This compressor is a very standard opto compressor except that it uses a ‘sum’ signal to drive the sidechain.
If the incoming L/R audio signal is reverse polarity (out of phase) then the ‘sum’ signal is zero or very close to it and there will be no compression…. but this also means that the compression meter will not work.
If you have one of these compressors and there is no compression happening BUT the meter is working, then there is a fault.
I have had several of these recently and the fault is often the LED driver transistors going open circuit.
But, once again, be careful because the compression with the SC2.2 is extremely gentle and unobtrusive….. it is quite hard to make it over-compress!
These analog compressors are much better behaved than the nasty digital ones that everyone uses nowadays.