Same button to open and close the unit, right? If it always closes, you can rule out the button. That would seem to point to either a sensor or the motor as TLS suggested. There is some electronics between the button and motor but I think it's unlikely that the electronics would cause an intermittent issue. Usually the logic circuits would fail completely. I would make sure that the transport moves freely and smoothly in both directions. If the belt is the issue, you should see/feel the motor's pulley spin freely but from the photo the belt tension looks to be ok. Need to get a scope on the motor wiring to check for a signal. If you have a digital multimeter with a locking feature that locks in peak voltage, you can measure for voltage being applied to the motor and even if it is only briefly the locking feature will show whether a pulse was applied or not.
