Brake Rivet Problem

I am trying to replace the brake linings on my Cherokee. I have had new
linings and brass rivets for some time now, along with a rivet tool.
The problem I'm having is that the rivets aren't forming a decent head,
but are simply being "squashed" more or less. They are spreading
slightly, so the result is a tight fit, but I don't think that they're
spreading enough to form an adequate head.

The holes in the backing plates do not look oversized, and I am using a
brake rivet tool, so I'm not sure of the problem.

Any suggestions?


Thanks,
JKG