Transparency and windowed controls

Hi all,

I am using a transparent component that works OK, but when painted
over some TWinControl descendants controls it gets overlayed. However
with other TGraphicControl descendants there is no problem at all.

In addition, I think only there are problems with windowed controls
whose parent window differs from the transparent-components one.

Do you please have any advice or track to go ahead?
Yours kindly,
Laura