I have the same problem:
Some times the filament just don't print in the first seconds, so the circle is not completed. Then, like Lucasz_b_g said, it fails because the raft starts peeling up, in the place where the extruder first starts printing.
It wold be good if Skirt and Raft were independent.
Thanks
Well, that makes good sense not to use both, because thats just Waste of filament.
I had same question, until i realized that there was no logic to use both.