As others have said, it should easily hold a piece of paper to the grill when it's running.
It's hard to say why it's not working without seeing it so you get to be our eyes. I would suggest turning the power off to it then pulling out the motor assembly. Clean the blower wheel and while it's out check the back draft damper to make sure it's not stuck closed. I've seen people screw them closed by accident when installing them. And the blowers, they always get dirty which reduces your CFM that it moves.
I've also seen the exhaust hoods get stuck closed and in one case even covered over by a window (on a tower) :/ two suites side by side were installed like that. The window section was decoratively painted so you couldn't see through it. I couldn't believe what I found when I found it.
If it used to work but simply doesn't now then I would look for a blockage somewhere. Make sure bushes aren't growing into the hood blocking the backdraft damper from opening or something similar.
If the motor has a capacitor then you can check that but only if you know how to safely do that. If you don't know how then don't touch it, they can be very dangerous if you don't know what you are doing!
Bushings can be getting dry if the fan is old. They may be serviceable but if you are noticing poor performance from it due to that then it is probably too late to save it. Re-oiling the bushings may buy you some time before you have to replace it though... If you are paying someone to fix it and it's due to stuff bushings then just have it replaced and save yourself the second service call down the road.
Let us know what you find or decide to do. We always like to find out how the story ends
