When he was dancing to Lucifer, Luhan’s mic/earpiece fell off. He had to dance while holding on to the mic, and the wire restricted his movements. He couldn’t hear the music from his earpiece, so he had to listen to the echo of the music in the dome and predict the timing of his movements, all for very difficult choreography that he only had a few days to learn and practice. That’s why he wasn’t able to extend his arm and his timing was off at parts. He didn’t smile even after the performance ended. 

The only bright side to this is that the wire kept pulling his shirt up so we got to see tummy!