MDI child do not arrange/cascade/tile

I have written an MDI application using the API and it works well with one exception:

I cannot arrange the childs with arrange/tile/cascade. At first I thought it might have to do with my hampering in the style properties, but after undoing this I have to say this is not the case. The only other unusual thing I did was to position dialog windows (as child windows) into the MDI children and fixing the size of the mdi children with SetWindowPos.

Has anybody got an idea why this does'nt work?

Thanks!

Horst