The belt on the dryer stops when going the other way?

I have a Simpson Scirroco 500. I've taken the dryer off the wall, took off the back grey panel that protects the belt. I also pressed the red button, just in case. I put a small load of dry clothes in to test what is wrong. When I turned the dryer on I could hear the engine going, but the belt didn't move. I gave the belt a little tug and that sort of "kick started" it. But when the dryer went to go the other way, it stopped. As if the engine wasn't powerful enough to do its job. Before I open up the back panel of the dryer to investigate the engine, could it be just the rubber dryer belt?
BTW: I always attempt to fix my own household items to save money, so I consider myself kind of a handyman. But this time, I'm not sure, as I can't find anything on the net that covers this.

Susan MH
March 2015