Zero Turn Mowers

Having an issue keeping the belt on the pulleys when dropping the deck.  Mows fine with the deck fully raised, but as soon as I drop the deck even one notch all hell breaks loose.   I started checking the level of the mower deck, when noticed only one side had the deck lift arm spring in place.  Purchased the mower used, which in it's first couple months did great.  But if only had one lift arm spring, perhaps it's lost some of it's strength and is bouncy enough to cause the belts to wobble off.  It's a big enough spring you'd think I'd notice it came off, so am guessing it wasn't there to begin with.  Could this be a root cause of the issue?  I've all ready replaced the idler pulley, probably unnecessary but it was the first best SWAG at the time.

Thanks

JR

Thanks