First off, I use PS7, and I have a question regarding templates/erasing/transparency.

I know the basics of using templates and such, but I just tried to make an icon using a template, and when I tried to erase the base picture using the pencil tool, some pixels would not erase at all. I tried to erase a larger groups of pixels and then using the history brush to restore what I wanted. The pixels that wouldn't erase were all in the same row of pixels. I was just wondering if this is just a glitch in the program, or is there something I can change so the pixels erase when I want them to?

Thanks in advance.