When you first receive the holdfasts take some coarse sandpaper (80-100 or so grit) and go round and round the stems. Round and round NOT up and down. This will remove any grease from manufacture but more importantly scratch a series of fine lines around the stems which really real help the holdfasts to grip.
The holes in the workbenches should always be 3/4" - which is just about the standard hole size for most workbench accessories.
To tighten a holdfast so that it clamps whack the top of the crook.
To loosen whack the back of the stem while pulling upon the crook with your free hand.
The way holdfasts work is that when hammered down to clamp they are just loose enough in the hole in the bench top so that they they wedge in. If the the angle the stem of the holdfast makes as it wedges in the hole is too shallow you won't get a good wedging action. Too loose in the hole, the wedging angle will be too steep to grip.
If the bench is thin and the holdfasts don't really want to grip just attach a bit of wood under the bench when the holes are, drill through, and you have effectively made the bench thicker where the holdfast holes are.
For a very thick bench, counter-boring the holes from the bottom work great. All you need is a little relief, so the effective bench thickness become 2 1/2" or so.