Walk Behind Lawn Mowers

I have a walk behind self propelled Husqvarna 7021r/961430063 that has almost quit pulling. The transmission tilts to tension the belt and I have tried every possible adjust that can be done. I have replaced the belt and it did no good. When you engage the drive it the belt moves, but the it slips on the pulley on the transmission. It is like the belt isn't tight enough. It will pull sometimes on level ground. It looks like the transmission should tilt further back to tighten the belt more or there is some kind of tensioner missing. It does look like there was something bolted to the frame where the belt travels between the engine and transmission.