Keep in mind, too -
These mechs use a series of weights and bounces to help weigh out slugs.
If your game has a steep incline to it, it could cause the mech in the door to not sit horizontal enough, causing a quarter to bounce the wrong way. Seen it happen a lot back when I was an operator.
If this is the case, you can test it by taking the mech out of the door, holding it upright, and try sending a few quarters through as you adjust the angle. If it doses start rejecting them, then some adjustment is necessary.