Also have you taken gravity readings? That will help a lot, it's how I fixed my problem.
2 of the 4 had the correct starting and final gravity, so I was thinking it had to be something else.
Pls ignore this reply as it was put in the wrong post I do appoligise
Exactly what temps are better? I have had the same problem fermenting at a constant 68 - is this too high? Not sure how to get temps down consistently...any ideas?