Press and hold for accents does not work everytime
When it stops working, does ps -ef|grep PressAndHold include a line for /System/Library/Input Methods/PressAndHold.app/Contents/MacOS/PressAndHold? If it doesn't, try to run open -a PressAndHold. Or if it does, killall PressAndHold;open -a PressAndHold might also be worth a shot.

You could also try to delete ~/Library/Preferences/com.apple.HIToolbox.plist. Normally defaults read com.apple.HIToolbox should show com.apple.PressAndHold under AppleEnabledInputSources and AppleSelectedInputSources.