A couple of days ago I ran into an executable called "carckme" which was a GUI application. So I decided to figure out the serial number it generates. Properly I found out the function but when it came to debugging I had some problems with IDA pro. Everything seemed okay but it didn't stop executing the app at the breakpoint. I guess it could be because of multithreading, as you know GUI applications are implemented multithreaded.
I came up with this issue because I couldn't really find the answer, I was wondering to ask here, at first to get to know the reason, and secondly to ask you to find and try to tackle the problem by issuing a way to make it solved.