Proform Tread slipping?

I have a 6 yr old Proform treadmill. The tread is slipping. I can hold it down with my foot while the treadmill is running. It had been rubbing on the right side because of a crack on the end cap. I fixed that for a TON cheaper then the repair guy wanted.
The tread has a wear mark on the right edge. But what is making it slip? I would like to fix it but do not know what keeps the tread from slipping like that.
Is this a give it away and get a new treadmill or its easy to fix issue?
Thanks