Again I am not the best at explaining but I would recommend possibly retouching up the solder joints as I just ran into a similar situation. I gave an express to my son in law, he soldered it up last year and it worked great . He pulled it out this year and 2 channels didn't work..I had him send to to me and did the norm, replace triac, swap pic etc. Nothing as a last resort I reflowed the connection blocks darn if that didn't fix the 2 channels. The solder looked good originally but it could have been brittle from over heating or not flowed good enough (guessing) Long story short try to touch up solder joints on #4 . The only other thing would be to check the actual connection block. Maybe there is a flaw in it. You have 12v but you need data also 12V, ground, and data. I think that is the correct lingo. I hope that helps you just ideas
Joe