Betta Fish: Betta won’t flare on one side

this is ginkgo! i've had him since june and this is the fist time he's never flared both gills. he's been like this for around a week now. he doesn't look injured from what i can see. he's constantly resting on a piece of wood so maybe he hurt himself that way? he's still eating normally. he does hide a lot though but he's always been like that. mostly wondering if i need to medicate for this since it's new or just let it be