Well in lieu of disassembling the whole board, I would pull the "b" keycap and shoot into the mechanism with compressed air.
I am always leery of compressed air - if there is debris you can force it farther in. I would always recommend turning it upside-down for a vacuuming first.
Another technique - and I am loathe to suggest it, for fear of what can happen - if the pivot plate is slightly out of its track, it is possible (last resort, and be careful) to use a toothpick or piece of wire and press straight down, then gently scoot it side-to-side to see if it will re-seat itself. Sometimes one of the "ears" will climb partially out of its groove, and you can get it back in place this way.