Thanks. I think you're right. I was hoping for an easier solution, but I'm probably going to have to start taking it apart.
Sounds like an upright - they're pretty simple. Also, if you've never replaced the belt, replace it. They actually need to be replaced every 6 months or year. You don't notice the tension decreasing and the brushes spinning slower, but if you put a new belt in, it will scare you the first time you turn it on and it pulverizes the floor with fury. Clean the hair/twine/etc off the brush while you've got it open, and if you can pull an end cap off the beater bar, there's usually some hair and such twisted in there too. At least in our house.
I'm guessing the blower motor is in the main body somewhere and is what produces the suction. I can also imagine a scenario where the brush belt works but the suction doesn't.
It's somewhere in there. You probably don't have to go terribly far in, and I'd be surprised if the blower motor was your problem.
However, before you go in, you are testing suction with the dirt canister fully installed and sealed, right? They don't suck at all without that connected.
I've got a broadly similar one, and airflow comes through the brushes and bottom, up the flexible hose into the dirt canister, dumping dirt there. Coming out the top, via the filter, air continues in and down to the motor in the base, which has the blower fan and also a shaft sticking out that turns the belt for the beater bar, then the air gets dumped out the remaining filters.
If the canister isn't installed, you have zero path between the hose and the motor, so you'll get no suction.
I would be stunned if there's a clog between the first filter in the swirl chamber and the blower motor, but you should be able to get your hand in there, or remove the filter canister somehow, and check. They come apart pretty easily.
