I asked on here about the same problem and as a result of the advice received, I have bottled the beer from the PB with more sugar to see if I can get carbonation. I did a plastic bottle at the same time so I can see/feel carbonation, and it is looking promising.
I’m a King Keg user and have formed the opinion that, while I accept a lot of King Keg users have trouble free use, for me while the barrel blow moulding is a robust item, the cap arrangement is the achilles heel. In particular the S30 valve with it’s rubber bands can prove problematic. To improve your brewing experience I would recommend the following:
Replace the S30 valve with a 15 psi safety valve (cost around £8)
Use an alternative non-return valve for CO2 like a corny gas post (cost around £8) or a Schrader valve (cost around £3)
Fit a 0-30 psi pressure gauge (cost around £10) to take the guess work out of leak testing and checking on the progress of carbonation.