Shop vac weak suction. new filter has worked.

Symptoms of a clog
Low Suction/No Suction
No suction at Inlet

Tools required
Wet/Dry Vac or vacuum with decent suction and a hose
Phillips Head Screw Driver
Metal Coat hanger (straightened out)
Flashlight

First things, first, make sure your canister is empty at the unit. I know, most people would have already checked this but I have been to a lot of client's homes and this was the issue. They were full or almost full. So just check it before you go to do anymore diagnosing. I suggest emptying it anyways, this way you can see what you clogged in your unit

Now if your machine has a utility valve on the unit turn on the manual over ride switch on and lift the utility valve and if you have suction your machine is fine, you have a clog. If you have no suction or poor suction
-Canister not attached properly
-Filter clogged
-There is a disconnect of pipe within your walls (I've seen it and it renders your system useless and is going to be expensive to repair the system)
-Your motor is going in your system (Your vacuum should sound like grinding metal and chances are it's throwing sparks and smells of burning electrical, turn the unit off immediately and unplug power cord) Your unit will need a new motor or you will need to replaced the whole unit. Sometimes can be cheaper to replace the unit.

So now back on track here, we are going to say the vacuum has great suction and is running smoothly.

Check your hose for clogs, easiest ways are to plug the hose into the utility valve and turn on the machine manually at the switch.
If you do not have a utility valve on your unit (this is where the other vacuum comes into play) plug your central vacuum hose on the end of the shop vac hose. Most hoses are standard and slip right on over the end of a shop vac's hose end (1 1/4 inch end). Turn on your shop vac and feel the suction at the end. It may not be as powerful as your CV but it should be comparable.

If the hose checks out to be free and clear we have a clogged system.

Now lets move to the inlets throwout your home, first we are going to plug our hose into the inlet that is nearest to the unit. If there is little or no suction we know there is an issue either in the distance between the nearest inlet or there is an open/broken valve.

Open/Broken valve (CV will not provide optimum suction with a open/broken valve)
-Plug the hose in and activate unit
-Check all inlets for proper seal, you will hear an annoying whistle or you will hear a gust of air from an improper inlet door closure. (Do this as quick as possible, if it is a clog you are putting strain on your motor, but if you vacuum on a regular basis chances are you know if there are whistling noises or missing pieces of an inlet. If you are missing an inlet door, replace the inlet. If all is checked out to be good, lets go back to unclogging again.

If there is suction at this inlet you know there is no clog between this inlet in the unit, try all inlets from nearest to farthest away. If you try your next inlet and have weak to no suction you found your point of a clog. This is where you will start to unclog your unit.

Take the hose end of your shop vac and insert it into your inlet. Turn the shop vac on and remove the hose quickly to create a seal and lose a seal. You should start to hear items moving through the line.

Now take your CV hose and insert it into this inlet, activate your CV and check suction if there is more suction, your slowly making progress.

Take a flashlight and look into the inlet to see if you can now see the obstruction. If you can, get your coat hanger out and go fishing. It may be easier to remove the inlet from the wall. Your basic valve is safe to remove. If you have a supervalve, do not remove, there is high current installed in the upper portion of the inlet for your power nozzle.

Repeat the Shop Vac suction and stop and the CV suction and stop until you clear your items.

This is going to take a few minutes of tugging between two vacuums. Eventually it will free if it went in, it has to come out. At the end of the day your basically playing tug of war between your Shop Vac and CV. But if all goes to plan, the clog will end up in your shop vac or in your CV unit.

Good Luck!