many bettas have different personality. I'm guessing Jagger is a mellow betta? probably the reason he doesn't usually flare up. Have you ever played w/ your betta(s)? If not, use the back of a ballpoint pen (the ones with the plastic casing and colored end) and slowly stick it in front of his nose from the outside of the bowl. If he follows it, he'll probably flare up. If you do this regularily, he may develop a better relationship with you and flare up more often without the pen.
Mic is the only Betta I have who flares a lot and he's a CT. He's very peaceful with his cories, but he does like to flare at just about everything else. He flares at the gravel vacuum, the algae scraper, and the algae disc I drop in for the cories (he flares then chases it and bumps it around a few times before coming to see if I have any "real" food) He also loves to chase my finger around the surface of the water and try to nip at it.
Toby (veil tail) rarely flares and neither did Ianto when I had him. I suppose their fins are probably just too heavy to bother with a full flare.