It sounds like you are using the same amp & footswitch I have.
Did this work previously, or has it never worked for you?
Is it the FS-4 footswitch unit you are using with the four switches marked valve, EQ, Compression, & sub Octave?
Are you using a high quality shielded TS cable to connect the footswitch to the amp (the leftmost socket on the back of the amp)?
The footswitch is supplied with a 2.5m shielded TS cable - if you are using that one, check it isn't faulty.
Try other cables