Win32 API and Enumerating Window Child

Question

You have discussed earlier in "Working With Win32 API In .NET" topic how to enumerate windows child and get there content but it seem that it didn't work with me as I think. I've run that code over some different apps like Internet explorer, Windows Explorer, Microsoft Word and WordPad and an App I've create with C# 4.5 and it work with the last one as intend but not work with the others. Could any one help me please and explain why this occur and if there is something I could do to achieve this goal ("Enumerating every child and get there content in any App running under Windows OS").