how can I fix my craft. mod#377070?

I can not get the drive to engage by using the control bar. I have changed the a) belt (that was fairly new) and b) a new drive cable unit ( that was also fairly new) but it still does not engage the rear wheels. While squeezing the control bar and with the mower running, I push the idler arm assembly using a stick, to put more tension on the belt just to see what would happen and the it does (yes) engage the rear wheels and the mower starts to move. All parts are clean and springs are good. Mower was running fine and then it just stopped engaging for no obvious reason.