IDA pro doesn't stop on breakpoints with GUI based applications

Question

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.

Answer 1

Everything seemed okay but it didn't stop executing the app at the breakpoint. I guess it could be because of multithreading

No, it's unrelated to whether or not the process is multithreaded.

If you set a breakpoint and the debugger didn't stop at your breakpoint, it's because the instruction at that address never got executed or the breakpoint got removed. It's hard to guess which of these happened or why without more context.