TransparentStretchBLT in Graphics.pas

I'm using TransparentStretchBLT, defined in graphics.pas, and it works
fine most of the time. But every once in a while it doesn't come out
transparent. For example I cut and Paste something and paste it in with
TransparentStretchBLT and 4 out of 5 times it works. the 5th time it is
not transparent. It's not always at the same interval, but it happens
everytime I use it.
For the mask color I've tried both clnone and clwhite and they both get
the same results.
Any thoughts?
Sim