Yes I agree this is a separate question about picks and rejects. Have you tried switching from grid view to survey view and then using the keyboard P and X keys.
Sorry, I have unlocked the original thread.
I will attempt that switch the next time I encounter the P/X keys not working. I was unaware of Survey View, as I'm still navigating all the options available in Lightroom (I'm a recent Picasa ex-pat).
Given your suggestion for Survey View, I found this YouTube video which quickly explains Survey and the shortcut keys associated with it; it looks very useful, so thanks for recommending it.
Anticipating this not being the solution though, my worry is that sometimes keyboard controls just fizz out, and quit working. Someone recommended deleting the preference file as a possible solution, which I will also try if the Survey View switch doesn't kick back in keyboard control.
Thank you for these suggestions.