Given the symptom, I bet this is gonna be "Stupid easy" when you find it.
And I bet REAL MONEY, it will be a "Ah, I see what happened, makes sense now" kind of moment. In 40 years in electronics, I've gone thru this COUNTLESS times, either due to what I did or what someone else did. Working in the shop, we would often say to one another, "You wanna look at this, I can't see what I did..." Usually, a second tech will find "it" in less than five minutes, even working on a complex multichannel radio site.

It's almost gotta be a bent pin, broken/fractured wire or similar. DO NOT make it harder than it is, but it's human nature to do so...........