Purple Plexi 800 - gain not working

[uki]

Hey guys, I've made this pedal few years ago and recently noticed that the gain is not working, there is distortion but from zero to max no change, it is on a constant level without change when turning the pot.
Tone and volume works.
I did a check on the pot(gain), cap(c6) and resistor(r4) those read ok in terms of value.
What is going on?



Follow layout and schem.

Thanks in advance.

[uki]

I got pics of the guts !