Well, if you've tried both the NO and the NC with the ground, then your volt meter should register on ONE of those without the coin even falling trough.
(1) Is your volt meter set to "continuity'? (you're not really looking for a voltage reading here)
(2) When you touch the ground (common) and the NC with your multimeter, does it indicate that it's connected by beeping or flashing?
If your meter is too slow, you might not 'see' a reaction when the coin actually falls through because the switch is very fast. But you shoud be able to test the switch and/or your meter by trying the NC and/or attaching it to the NO and COM and then pulling down on the wire that is hit by the coins as they fall.