To really try and discover the problem and an answer, everyone will need to know:
What weight rod are you using?
Are you dry fly fishing, nymph fishing, streamer fishing, etc?
Are you fishing with weighted flies?
What size flies are you fishing?
Are you using a thread furled leader, fluorocarbon furled leader or mono furled leader?
What length furled leaders are you using?
I make and use my own furled leaders and I make all mine from 4 pound test Berkley Vanish fluorocarbon. They are all the 10-8-6 formula and they are all 7 foot long. I use these on my 4 weight, 5 weight and 6 weight and have no problems with the turn over of the tippet or fly. I attach 5 to 6 feet of 6 pound test fluorocarbon to the end of the furled leader. I hardly ever fish with dry flies. I use this set-up for my bluegill/bass poppers, weighted nymphs, streamers and buggers and have no problems.
Warren
Fly fishing and fly tying are two things that I do, and when I am doing them, they are the only 2 things I think about. They clear my mind.