Steelhead fishing in Erie tribs using Berkley 6# fluorocarbon as 2x tippet also Maxims Ultragreen fluorocarbon leader material as 2x tippet. Problem of knots not holding up, used improved clinch knot & Pitzen knot. I make sure they are tight. Attach tippet to end of small swivel that knot hasn't come undone just where attached to fly. Should I buy regular fluorocarbon tippe t 2x or try a different knot?