Did you look at the end of the line? A little curl there indicates your knot failed. Try breaking the line without a knot in it - the line could be bad.
I have tied an improved clinch knot for about 45 years and never had a problem with it until I started using 12 pound PLine Flourocarbon. I wrap the line 7 times and it has always held. Started breaking the fluro on the hookset and it would break when I tied the knot sometimes. Switched to the palomar and no more problems with it. I still tie the improved clinch on all other lines, including 15 pound PLine flouro - never had a problem with the higher test lines. I can tie the improved clinch faster and better - more practice.