If you haven't already, now is a really good time to check your buttons and clean them, to make sure they work properly whilst you're in there (before you put the board in preferably, I happened to only take the photos afterwards - writing guides is hard!)

You'll see on each of the buttons on the casing that there is a little circlip on each, that should sit in a detent - double check/verify that it's where it should be, as otherwise this can make the button hard to press and not return to its normal position as well. There's also a write up here on iFixit that's great for the buttons.