"B" Button only works when pressed HARD.

I've gotten it torn apart according to IFIXIT's ABXY replacement guide, and I've cleaned each individual button as well as the contacts on the motherboard with isopropyl alcohol, but... Well they didn't really seem dirty in the slightest. The buttons themselves had a very small amount of gunk on them, the contacts looked perfect already. I don't see any physical damage...

EDIT: I should mention that this happened suddenly. And just a though, could it be that I'd need to replace the ABXY part on the motherboard?

Any ideas before I put this back together and find that cleaning didn't make a difference?