Question about gain control in consoles

Hi.
I have an AMEK TAC B2 console. The gain on some channels is a little ruff, sometimes I have to turn it all the way up so it will start passing the signal. It's like an obstructed pipe that has to be freed with a massive blow off air.
Does anyone know why?
The console is from around 1984, so it could be a cap. Otherwise it sounds all right, no noise whatsoever.

Thanks in advance for your help.