This is a recurring problem with some of those early compressors…. the pots wear slightly and the balance goes out.
There’s no complete answer, there will always be the possibility of a slight imbalance, frankly, the quick way out is to adjust the gains either before or after the compressor.
Coincidentally I have had another SC2.2 in for attention yesterday….. this model was one of the best in design and they are generally stable for gain.
If it gets too bad I can fix it for you by replacing the input and output pots. The cost would be my standard £25 plus the cost of return to you.