A coin is placed on a silver coaster covered with black velvet, a glass is placed over it, and a smaller coin then penetrates the bottom of the glass. Unfortunately, I received the trick without instructions. Only in about 14 out of 15 cases can you see the small coin appear under the glass. In exceptional cases, the small coin unfortunately remains under the large coin. Is it possible to make the trick work every time? Thanx for any help!

I got help, thank you very much.

What was the answer to getting it every time?
Giving it a whacker from underneath?

The instructions stated that a straight surface is important. Maybe the trick has to be performed without a micro magic mat/close up pad. The instructions said the rim of the glass should fit exactly around the edge of the velvet surface. Of course it has to crack when you press down the bottom. The nice magician who helped me wrote that the trick always works for him.