I've tried everything I could think of to clean the brush on my Shark Navigator Freestyle, but the brush stops spinning still. What else should I do to try and solve this problem?
I do not have that particular model in front of me, but here is a likely problem.
It could be the brush bearings/ bushings are clogged with dirt or hair, or even rust.
If you can remove the brush and the bearings/bushings ( which are the parts that hold the spinning brush), see if they work freely in your hand.
If one end or the other is bound up, it is clogged with something.
If you cannot remove the bearings/bushings from the machine, you might try some drops of a penetrating oil into those bearings/bushings to see if they will "free up".
I personally use a product called "P B Blaster", which, although it has a strong smell, is one of the best penetrating oils on the market.
After re-reading your letter, I am wondering if the belt is the problem. Although it has a "lifetime" guarantee, that might only mean it will be replaced by the company for free should it fail.
If it is worn out, it could slip, causing this problem.
God bless your efforts.