Tube biasing problem

Hi guys, i'm not a newbie in the world of audio, but somewhat a newbie
with tubes. A few months back I took delivery of a beautiful CJ tube amp. It
is rated a t 100 watts per channel, and has 2 sets of quad el34's. Being this is an older amp, the tubes that I received with it are also at the end of there life span. They are not matching quads neither. One set is Svetlana, and the other setare JJ's

My question, just recently I am not able to bias 3 of the tubes. CJ has the biasing pots with the leds which works beautifully. At first I thought there was a problem with the pots, but when I rotate the tubes, the 3 tubes that I was not able to bias before still cannot be biased in different pots. The properly working tubes can be biased in any of the pots so I ruled out the amp itself, and figure it has to be the tubes. Is this normal for a tube to resist biasing at the end of it's life span? As far as the amp performance, I am still playing music, and everything seems to be working properly, and there is no noise coming through the speakers. Any help from experienced users would be greatly appreciated. Thank you