Are you sure you are in the same color depth on both machine?
For exemple if on one your in a 16bit/color mode (655535 colors) and on the other one in a 24bit/mode (millions of colors) , it couls make the difference.
Because technically, depending how the FE is coded , if it use a "Transparent color" , if that color is defined in 16bit or 24bit, in one case the color can be considered as transparent in the other not.