I don't seem to be having good luck with the Fixcut head that I got for my Stihl trimmer. The trimmer runs fine, but with the replacement head on it just doesn't seem to be cutting worth a darn. As best I can tell the problem is that the Fixcut head is sufficiently loose that it slows down as soon as the line starts making contact with the grass. I don't know what to do about this except to torque down more, but the manual specifically warns against overtightening. I am probably making a bonehead mistake here, but what bonehead mistake?