Anyone here good with Lawnmowers?

I've got a self propelled mower, that won't self propell. It's been this way for probably three years and no matter how much searching I do, I can't seem to find a reason as to why it wouldn't be working. Anyone have any ideas at all as to why it wouldn't go on its own, or a way to fix it? Mower is about 4 years old, it's a Lawn Boy 20in, non variable speed, with a B&S motor. I don' have the model number in front of me.
The mower still works as is, but since I paid for a self propelled, I'd at least like to get it working again.

 

I have issues with twigs getting stuck and causing the self propelled to stop working or popping the belt off.

 

If the belt is still working, the next thing to check, would be the gear that drives the wheels.
Usually, the gear teeth, on the inside of the front wheel, that the drive gear engages with, will break, before the drive gear.
If so, you should be able to order a new wheel, for less than 30 bux.
At least, that`s the way they used to work. Not sure, if they are still made that way, or not.
Hope that helps.