Can’t deepthroat but not because of gag reflex? Help!

Okay, so my boyfriend of a year and a half, we’re trying to spice things up with our sex life. And as of recently I’ve been giving him A LOT more blowjobs. I never used to really give them a lot...maybe 1 or 2 in like 2 weeks? But now, I give them to him 2-3 times when we hang out, and then we have sex once or twice throughout the day as well...but no matter how hard I try, I can not deep throat him. But the thing is, when I go as far as I can down, I don’t even gag or anything unless I stay there for a few seconds. It’s almost like his 🍆 physically can’t go any further down my throat...he’s about 8 1/2 inches, and I get down till I have like 2 inches, maybe 3 left...even when he pushes my head down as far as it’ll go, I don’t gag, I just can’t get myself to go down further on him...and I really want to because I know he’d LOVE if I could fit his whole thing into my mouth, and I love making him feel good, so I really wanna be able to do it...anyone have any tips or know why I can’t go down further on him?