This is a question I asked Sol Stone when I was about 14. The answer is that depending on your hands, you may not be able to do the same things as certain other people. Just a fact of life.
That said, if you are talking about Deep BackClip, then there is always supposed to be a coin visible in the hand that rests on the fingers directly above the backclipped coin.
If you are talking about standard back clips, near the outermost joints of the second and third fingers, then you have to learn how to sink the coin farther down between your fingers until it is just out of sight but is still held tightly enough so that it doesn't fall.
I'm not a coin worker by any means, however I've found that shifting the two fingers on either side of the coin up and down VERY slightly works it down just enough.