Random question: why won’t it dry?

So, I’ve been trying for about six months to stop biting my nails (which I literally used to bite until they bled, rip off the top layers, and chew until my teeth could not longer find any bits to gnaw on) and what seems to help me the most is repeatedly and copiously painting them so that the corners and edges don’t bother me so much.

Why is is that if I paint one thin layer of nail polish on my nails (clear basecoat) that it hardens like it should, but if the layer is too thick, it never hardens and later gets marred by something and I have to peel it off in a soft layer? It does not matter how long I have let them dry.

Also, what is the deal with the colored polish? That never dries hard, even if I put super thin layers on and then cover it with a top coat.

Maybe someone who knows something about paint can tell me why this is?