Look at what Jcbtubes has suggested and if that is not it, I have seen the adjustable pot cause some problems.
Check it is the same value end to end(compare to the other ones which are OK) that is along the conductive track - the two outer tags from memory. Also from memory they are 10K pots, so that is what you should get.
Also put you meter from the wiper to one end and check there are no sudden jumps in value. Compare with the good ones again.
This is what showed up as wrong in my amp.
New output tubes, new pots (I replaced all of them just in case) and a new resistor - still running well 6 years (and another set of output tubes) later.
Check it is the same value end to end(compare to the other ones which are OK) that is along the conductive track - the two outer tags from memory. Also from memory they are 10K pots, so that is what you should get.
Also put you meter from the wiper to one end and check there are no sudden jumps in value. Compare with the good ones again.
This is what showed up as wrong in my amp.
New output tubes, new pots (I replaced all of them just in case) and a new resistor - still running well 6 years (and another set of output tubes) later.