QPSK in benchmark

Hi everyone,
I’m using gnu radio benchmark to do experiment. I want to test under
different modulation, BPSK, DBPSK, DQPSK all works well, but there seems
to be some problem in QPSK. I also asked some lab mates, they also face
this problem.
Anyone come across this problem, how did you deal with it?
Thanks in advance!
Ada