The debugger has been giving me troubles since 9i. I think Oracle's debugger is buggy. For me, it hangs.
I've found it helps if I uncheck the "Update call stack after each step" preference. If a package calls another package it still sometimes hangs on me.
I guess it's worth mentioning to make sure that the "debug information" has been added to the package just to make sure.
Anyway, I've sort of given up on the debugger. I've been convinced that it's worth it to instrument my code and that's how I do it now.