Hi Larry
My bench is 3 1/2" European Oak. I use the Gramercy holdfasts. They are perfect. Never slip. They did not slip in my previous bench either, and that one was 1-1/2" MDF with a 3/4" layer of Karri. The common denominator was that the holes were drilled with a brace and bit ...
I repeat - the holdfasts never slip. I wack them with a 18oz UHMW-headed mallet ..
Look closely below, and you can see a slight gap around the shafts ...
They unseat with a rap. No drama.
The design of the Gramercy holdfasts is just fine, thank you. The issue is the hole in which yours are seated: the walls are too smooth or the hole is a little too tight.
Instead of sanding the holdfast, take a rasp to the holes and roughen them up. Them will widen them a smidgeon as well. That is probably all that is needed.
Regards from Perth
Derek