The last time this happened, the WT developer didn't realize their source frame was black. You're out there and know who you are ...
Can you verify the content of the source frame? Are you displaying the contents of lastError() if the call fails?
Did you try the sample code? If not, I suggest you populate the frame buffer in the sample program with dummy code and run that. This should work, and then you can compare the sample code against yours.
What versions of the client and server are you running?