Middel seatbelt will not loosen

the mechanism is jammed. best option would be to replace it. there are many available used

the channel inside the rear chair teeths worn out fail stucks at a positiion

I agree with the above comments. I just had this same problem. Holden (Australia) quoted me AU$650 to fix it. I bought a T50 torx bit (like an Allen key)from supercheap auto for $11.
raise the seat back up as much as you can (I could only raise mine about 20 inches). Wriggle your hand in and find the bottom bolt of the seat belt clip (this will be the seatbelt clip for the side seatbelt) and start undoing it. It isn't hard, just fiddly considering you probably won't be able to see what you are doing. Once undone, you will be able to raise the seat.
I was still unable to loosen my middle seatbelt immediately. I came back to it a while later and it had freed up.
Simply a matter of then bolting it back in place. It was so easy - and I'm just a middle aged woman with no handyman/car skills whatsoever!

On my 2016 Ford Escape the rear passenger seat folded down and would not hold back up the problem was that the seat belt buckle pivoted down below the level of the seat when I try to raise the back of the sheet the seat belt buckle would prevent it it ended up crushing the plastic on the seat belt buckle preventing it from working the hold the seat belt I was able to repair the seat belt buckle with a screwdriver regular Style and a cigarette lighter the soft in the plastic back up so it would stay in position

Same problem here...belt stuck and pulled so tight against the top of seat when in the upright position, I would have to keep seat in "down" position for fear the tension might cause permanent creases in leather. Had brother loosen the bolt located at the base of seatbelt where the back of seat and botton split (hard to get to and tough to loosen) which seemed to have solved issue but a few months later after using it to strap in niece's car seat it happen AGAIN!
Frustrated, had brother completely remove bolt rendering belt useless (fortunately backseat is almost never used anyway.)
I'm going to check for any safety recalls since this doesn't appear now to be a fluke or isolated problem w/ just mine.

If you find a fix or recall please let me know & I'll do the same.

Hey everyone--after searching in vain for an entire 10 days, and absolutely REFUSING to pay Ford's $100 diagnosis fee, and their $300 replacement suggestion ($400??), I decided to figure this out myself. And I did. Do the following to let loose the tension that has kept your seat stuck in the down position (though my truck is a 2008 Ford Escape, I have a strong feeling this is the case with any vehicle):

1. Remove the seat bottoms (which are folded forward and upward against the back of the front seats). To do this, find the bars that are bracketed into the floor. There may or may not be a little yellow lever on one of the two brackets. If so, flip it and slide the bar out. If it stops, pull the other side out. Doing this should allow you to completely remove the seat from the car. Repeat with other side.

Now you should have plenty of room to work. Move front seats forward, and you've got even more.

2. Remove your back seats entirely. (Sorry- it's a must.) For many, you will require either one or both of these:
One Torx T-50 socket to remove T-50 bolt (which bolts seat belt latches to the floor)
One Torx T-55 (which bolts the folding seat into the bracket that bolts seats to the floor).
One socket extension (tip: the extension gives you much more torque, and its easier to remove bolts) and
One a 5/8" socket. (See continuation after following tip.)

TIP: if you have a 40/60 split, you have to remove the 40 first! This will allow you room to move around the 60 seat and its stuck belt.

Continued:
Remove bolts that bolt bracket to car floor (this is the 40 seat). I believe there's two: 1 directly below the seat, and 1 that also bolts the 60 seat (you'll be using the 5/8" here)-- Don't hold me to it, though, as I've seen some different stuff. Pull seat out of the way to avoid potential damage. NOTE: THOSE BRACKETS ARE SHARP. WATCH THE LEATHER/SEAT MATERIAL-- AND YOUR HANDS.)

2. Remove bolts on the 60. Though the top part of your seat is down, you should be able to the bolt that keeps the bracket to the floor beneath where the seat cushion would be (and where the upper part of the seat is laying flat). A ratchet would probably be best for this since you can keep the socket in place and wind the thing completely out. I should say here that my seat couldn't have been tighter to the floor. If I got mine out, I'm sure you can, too.

Now here come the tricky, but verrrrry easy parts. They are vital.

1. Now that the brackets are off of the floor, the 60 seat should now be free-floating in your hands, but tethered by the T50 bolt that is keeping the seatbelt buckle to the floor. Remove it. Now you should be able to remove the seat altogether. I had to bring mine inside where it wasn't 15 degrees in my garage, but I believe you'll do these parts in the next five minutes. Do this next:

2. TIP the 60 SEAT UPRIGHT (just like it would if things were back to the normal operational mode). This is VITAL. If you cannot fix it with two or three tugs from here (don't waste more time than this-- the final part is coming), then do this next:

3. Find the latch on the back of the seat that locks onto the latch of the car. Using two gloved fingers (or tools), press the latch inward until it's in the LOCKED position (fooling it into thinking you've put it up and latched into into the truck bracket).
This should have also reset the button or lever that you used to unlock the seat in the first place. Now give your belt a tug. It should come loose like normal.

IMPORTANT NOTE: seatbelt tensioners rely on gravity (thing momentum during a crash) that locks the belt. If the seat is not sitting UPRIGHT (like in normal use mode), that mechanism is going to obey gravity if it is tilted backward or forward (which is why the seat got in the downward position to begin with) and lock.

That's it. Bolt the seats back in, and go buy yourself $400 worth of rewards.

there is mechanical lock on every seatbelt activated when you push the breaks find lock and realse it by hand

Lift up on the rear seat cushion and fold it towards the front. Now pull the lever and fold the seat down.

middle seat belt shouldn't have anything to do with it. The seat locks down for stowage in the rear cargo area. you can pull the release handle on the back of the seat to release it back into the up position.

make sure that the rear seats are properly locked into rear catches as this can stop your seatbels from working if ok then sounds like your seatbelt is faulty req new seatbelt.