Hello,
I have been using elvui since ages, and I always been lazy to fix this problem.
My Num5 is bind to "Displacer beast". everytime i log to the game, the game doesnt "recognize" this bottom AT ALL.
only after i /reload the game, the num5 bind is working.
I would go with what QuizTIC said. Make sure you are on the most recent build of ElvUI. I too use the NUM buttons and they all seem to be working just fine.