Hi Stevee,
I always use braid for hooklengths & don't have this problem
I use the knotless knot so the first thing I do is tie a single overhand loop for the hair stop to go through. Once I have a small loop I put my baiting needle in it & pull the hooklength really hard to tighten it fully. Trim the tag end & carry on making the rig. I use drennan sink braid which is very supple & works a treat
. Maybe try a dap of glue on the knot, but just try pulling it harder first. Good luck. You will have a few months to try it on some carp pools before you are back after the Barbel
Cheers
James