Problems with Debuging

Question

With one of my sites breakpoints dont work - the code runs without interuption.
However, if I run / start debugging (F5) the site and then Attach To Process (WebDev.WebServer.EXE), then the code breaks where I want it to.
The strange thing is that I dont experience this problem with other sites of mine, so it kinda suggests a site specific issue.
So, my question is how can I enable debugging without having to Attach To Process?
Thanks