I have to echo BettaxFishxCrazy else- temperant of the betta. I had a betta called Polkee and he wouldn't even flare his fins (which were gorgeous and orange and long <3). My plakat flares for about 2 seconds, then retreats into his den, then pops his head out, comes to the mirror, and repeats the sequence. While one of my crowntails flares all day at the bettas beside him.
I don't know if this would work for you, but some of my bettas don't flare at mirrors... they flare at CD's. I have absolutely no idea why. But maybe yours would flare at CD's too. You could try it.
In the hobby for 9 years.