This is a common problem.
I have to agree with Scott Lee and others that tapping the balls in is not the solution. As a matter of fact, it may very well be the cause of your problem and not the solution. It most certainly hurts your efforts as opposed to helping them.
Here's what I do.
On some tables the front ball will not sit directly on the spot, it will roll slightly to a "low" spot every time. As Scott says, let it go there, and then rack the other balls aroung it.
Also, there are equipment issues where that leave the front few balls floating around in the rack no matter how tight you squeeze them. If this happens, don't just squeeze so hard that you distort the rack. This may get them tight, but when you let up the rack just relaxes back to it's original position and pushes the balls loose again. The solution is to squeeze the rest of the balls together and then push them as a group up to the back of the 3 in front. By pushing them up from behind, you can get them all to touch. They will not, however, be touching the rack, but that is fine. You now have a tight rack for your opponent, or yourself.
Certaily not the only way to skin a cat, but it sure works good for me!
Royce Bunnell
www.obcues.com