Off printing cause by the loosen belt

Print 29 shows that the X axis idler belt must be loosen. As sometime screws can be loosen up due to the vibration during printing.

So I just tighten the belt, and come back on the following day to test but still it happened again, so I dissembled the cover and found out the both screws of the idler pulley drop out. Maybe I didn't tight the screw enough at the first place.

No choice I have to dissembled the Y-stage, took out the motor and assemble them back.
It solve the problem.