Hello! I went through this with my two sons. I feel your pain! I solved the problem by easing up on theory and playing "contest" pieces. Their piano teacher knew she was going to lose them if we didn't figure out something quick, so the remedy was to have them choose and play whatever they wanted with no restrictions. Sometimes they choose to read notes but sometimes they will choose to play by ear or even add on their own "twist" The fun came back! They now only choose all of the "popular" songs you hear on the radio
No more Bach or Beethoven Lol (at least not yet) They also started composing their own pieces. I'm happy to say the joy of playing piano is back! Good luck!
