Reply to S4 headphone cue mix knob problems - anyone had a similar problem before ?

Just hoping, has anyone had any problems with their S4 and the Cue Mix knob?

Got a new replacement S4 for my faulty one (after ages waiting) and sold it on to a guy just before xmas, and turns out the mix knob for his headphone mix is not having any affect and won't adjust. Everything else with the Pro 2 software seems to be ok.

Just wondering has anyone else had a similar problem and if it may be a software/driver/settings problem, or if it is a likely a hardware problem?

He's coming over in a day or so to double check with my system (I've still got Pro2 installed) and hoping if anyone has any suggestions we could try to remedy it ?