How many times have you had to move the motor to take up slack in the chain? Kinda sounds like moving the engine back to snug up the chain has made it so that the clutches on the belt drive aren't able to get any more grip. Have you considered just taking a link out of the chain? Easy to do.
This....put the engine back to where the clutch WORKS,....than adjust chain tension by subtracting links,than maybe adding a half link.
Belt is WAY too loose, as pictured
Luck.
GTC