Your right (Y) switch is working as you get both channels, and the A part of the left switch seems to be ok too, so it's probably the B switching on the left switch. Check the left-most centre lug on the left switch, as that sends the input signal to the B output.
It could also just be a dodgy switch with a bad contact, so either check it with a multimeter... or, if you're desperate, swap the left-most connections on the left switch to its unused middle pole lugs.
Alright I re-soldered the joint and now have continuity between the left middle lug and the mono jack tip.
Plugged it in and no luck. Just switching channel B on/off
Am I doing something stupid with the amps? Plugging into the wrong input? A/B mono jacks are for the amps right? and the far right jack is for the guitar because both chains lead to it (hot/ground)
Ok, I've just whipped these up quickly to show which points should be hot and ground in each position (red = hot, white = ground). You'll have to click the switches to get the 'bar' contacts on the correct side for each state before testing. That's assuming you have the switches oriented correctly so that the poles (sets of three lugs) are running across the switches/images. I've numbered the lugs so we have a common frame of reference.
The 'bars' represent the active switch contacts between lugs, the little squares just show if the other lugs should be hot or ground, and the large circles show the hot and ground on the selected jacks. To test, get the switch in the matching state, then put your multimeter probe on the red 'IN' circle and the other on the red A or B circle, as appropriate. If you don't get signal then move the A/B probe to the highest number red 'bar' lugs, then next highest etc until you find where the signal stops. Repeat with the white dots.
So, for this 'A channel on' state your hot signal should be going: red circle IN -> 4-5 -> 10-11 -> red circle A. Make sense?
As it's your 'B' channel that's the problem, this is the state you'll want to check. I think you've already checked the 2-3 lugs, so it may be the 7-8 contacts. Signal path: red circle IN -> 3-2 -> 7-8 -> red circle B.
The 'Y' states ignore the A/B switch and just use the Y switch to bridge the signal across both the A and B channels, but I've drawn it up showing the 'Y' from both the 'A on' and 'B on' states just in case.