Programmers don't guess!! They make insiteful observations based upon the information available. You need to understand and debug your code.

As Paul said

You're supposed to first diagnose the problem, see where it breaks down, read the documentation, and then add code to correct the problem. When it comes to Windows programming, you don't just add code just to see if it fixes something -- you need to know the exact reason why something fails first.
You've only posted a part of the code with which you are having problems. But as all parts of a WIN32 program work together with the OS calling your routines and your routines calling Windows routines, the problem could be anywhere.

You need to start with a known working example of a WIN32 program that uses a dialog box. Compile that and make sure it works as expected. Then add/change as required and after each step make sure the program still works. If it doesn't you then know that the problem lies with the last change.

You must understand how a WIN32 program is expected to work. This has to be learnt - it can't just be found by trying some ad-hoc code and hoping for the best.

I suggest you read, digest and understand

Programming Windows 5th Edition Book/CD Package: The definitive guide to the Win32 API (Microsoft Programming Series) Charles Petzold
http://www.amazon.co.uk/Programming-...9984984&sr=1-4

Win32 Programming (Advanced Windows) Rector & Newcomer
http://www.amazon.co.uk/Win32-Progra...9985057&sr=1-1