I use it all the time and rarely have break offs playing fish,
I use 0.17 (6lb 12oz for down the edge when fishing for lumps and have been done a couple of times when a monster has powered off and took the laccie to the limit but usually the line does the job intended,
And 0.13 (4lb 12oz) for up in the water for f1's and have landed that bunus lump,
Were You on the pole or rod and what diameter was You using.
TBH I once had some 1.1 and 1.3 that were bad but that was years ago. Mainly I use 0.15 and 0.17 and like others have not had problems with it,
Fist thing is I assume you are wetting the knot.
When I go for silvers with the odd chance of a better fish I use the same as silverman silstar match team in 0.10 and 0.12 its closer to the stated diameter than the preston.
For carp fishing upin the water ATM im still using the 0.12 although I will change to 0.15 preston soon.
Yeah it says Reflo Power or something on the box and I was wetting the knot when tying it.
The only other thing I can think of was maybe I didn't make as good a job on the knot because I tied it on the bank (although it's weird that it happened twice in a row). I've ordered some Shimano Silk Shock now so I'll try that and see how it goes and maybe I'll try some more of the Preston stuff in the future.
I've tried a lot of different hooklength lines - also tested their knot strength and diameters. Most are quite a bit thicker than claimed, and most break at less than the stated breaking strain.
The best that I've tested is Ultima Power Match - it's actually the diameter that is claimed, and has most of the claimed strength. Reflo Powerline and Stroft were all over 0.02mm thicker than stated - meaning that the 0.12mm mic'ed up at closer to 0.15mm, which is a huge difference.