[mrerick]

It is certainly possible that the magazine button is not pushing the hold open far enough up to ensure engagement with the slide. If it's just on the edge, removing upward pressure might allow the slide to slip off.

Double check the rear face of the hold open lug that engages the receiver, and the place on the receiver that it engages. It is not that unusual for the face of this part to be damaged.

This may be worn, chipped or in some other way damaged, allowing the receiver to slip off the hold open when upward pressure is removed by removing the magazine.

Also check to make sure that the very small leaf spring on the tip of the hold open is formed properly and has not been bent, snapped off or otherwise damaged.

Marc