These handle mechanisms are pretty easy to figure out. Usually a solenoid locks the handle so it can't be pulled down. When credits are bet, the solenoid releases and unlocks the handle. At the end of the handle pull, there is a lever that releases the mechanism so it can snap back to the original position. If this lever is stuck or interfered with, the handle will not pull back to original position. The issue is that the mechanism is behind the bill validator so it is difficult to get a good look. You may need to remove the validator cage and printer in order to get a good look at where it is stuck. If you have one of those endoscopic cameras, it may help.