I've experienced the same problem with one of my projects. The only things that I can see that I've done differently from other projects is that I've resized the original form, and also at one stage I had it calling a second form, which I later decided to remove from the project.
Given that it's possible to make the taskbar button appear by minimising, and it's still possible to switch to the program using Alt-Tab, this is more of an annoyance than a major problem, but I would still like to know how to fix it.