Caught 3 Steelhead yesterday, using 5x fluorocarbon tippet. Landing 2, I noticed my improved clinch knot failed. I did retrieve my flies before releasing fish. Is this a problem with fluorocarbon? Is there a better knot to use? I made sure I tied the knot the right way.
The line is no doubt slipping, you could try an overhand knot on the line first to draw the clinch knot down against. Have you considered a loop knot, using a double or even a triple overhand for the knot?
My guess is the knot is not getting seated properly. I went through this last year with the clinch knot. What I found was.... when I seated the knot I had a tendency to hold the tag end against the hook with my finger. What that did was create a gap once the tag was released which leads to the knot slipping. Now I make sure I wet the knot and draw it 1/2 tight and release the tag and then fully seat the knot. When done properly the tag should be at a 90 degree angle from the knot. I haven't had a knot failure since I figured out where I was going wrong. Also check your knot but giving your fly a tug after each fish to make sure your knot is still good. There are other knots you can try but the clinch knot is the fastest for me to tie.