Caps and rotors can eventually wear out, but they generally never stop working unless there's a physical failure (talking about good caps with brass contacts). If you had moisture inside the cap, I'd bet that was the issue. Drilling a few small holes around the perimeter of the cap can help, and won't hurt anything. I was also going to mention that you can get carbon build-up inside the caps. If you got a gray appearance on the rag when you wiped it out, that would have only made things worse, especially with the moisture.