Backfeeder fb not working

I've posted this problem a while back and thought I'd solved it but alas, the problem remains :(

The OD part of the pedal works but the feedback momentary switch doesn't produce feedback. When I was investigating this a while ago I had some success after I put my finger on a few sections of the reverse side of the board, roughly where the transistors are while the board was switched on. Suddenly I was able to hold the momentary switch and have the desired effect.

I reflowed the transistors and connections around that area but haven't had the same luck again. So I've finally built myself an audio probe and had a bit of a poke around but to be honest I don't really know what I'm looking for! I noticed each transistor seems to produce a tone in a different frequency range which I would assume means they are working. Could anyone give me some guidance on where I should prod and what I'm looking for? Sorry, I'm a bit of a noob but trying to become less of one with your help! :)
 
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