I think David Neighbors had some routines in which the coin wasn't stuck in the recess.

I'd like to thank everyone for their suggestions. The instructions that came with one of my Boston boxes recommended rubber cement, but when I went to use mine the entire jar had evaporated and dried up. I didn't know rubber cement could do that. Instead of buying another jar I decided to try the Blu-Tack many of you recommended. That's when I was reminded that we are living in the age of Amazon. No brick and mortar store I went to had it including Office Max. I thought it would be faster to pick it up somewhere but only Amazon had it. Long story short Blu-Tack works beautifully. Does anyone know if Blu-Tack dries up over time and becomes unusable?

I've had it at the bottom of a cup, covered by a dollar sized coin, and it has been there for years, with the same adhesive quality (for my Twilight Zone Wild Coin routine).

I get it at Dollar Tree and Walmart. It's also called Poster Tack, which is the more common name for it in the US.

I've had it dry out on walls after many years but the coin stuff I stuck together with it (4 or 5 years ago?) is still stuck together.