It could be a combination of the timing or simply the speed the rest falls away. I have a QAD Ultra Rest and shot for a long time with cock vane down without problems. I noticed recently it wasn't droping as fast and since I can't figure out how to get it apart to tighten the spring, I switched to cock vane up. I put a TT on my daughter's bow and, after making sure it was adjusted properly, she was able to shoot vane down. However, she shoots a pretty short arrow, so I have her shoot with the vane up just to eliminate any possible problem.

Have someone put some tape around your arrow when you have the bow at full draw - put the tape right in front of where the arrow rests on the TT forks. Then let down and watch how far away the forks are from the tape when the rest is first fully drawn up. If those forks are at their full height before the tape is with aninch or twoof them, you need to adjust the timing.