switch to a kimura grip and jerk the hand up and away, focusing on the area between their thumb and fingers. use your feet to keep the other arm from coming back to grab.
alternatively, armbar towards their head instead of their hips.
yes, they can sit up or backroll, but that's also because you're not applying enough counterpressure with your legs.
try not to armbar someone flat on their back. hump their tricep forward so they're on a shoulder and crossface the fuck out of them with your top leg.
people try to mosey their armbars from both top and bottom. that's fine, but you're gonna have more success hitting the armbar with speed. i'm not saying to FINISH with speed, but especially against someone who knows what they're doing you've gotta be mindful both of your position and how fast they're able to defend.
if you're trying to armbar someone bigger than you who's already got their arms together you're gonna have a bad time. as nuts as this sounds i almost give people the armbar from mount because i'm really good at sitting up as they fall back and even better at not letting the tip of my elbow get past their thigh.
if you're getting armbarred from guard, you're not controlling their posture and minding where your elbows are, which is are basically the second and third rules of being in guard (first is minding your posture.) don't stack someone on their shoulders unless you've got their hips controlled.
if you're trying to armbar from guard your best bet is either belly-down or a pendulum sweep. unless they're an absolute dipshit who doesn't know how to defend you're gonna have a real tough time.
tldnr: control posture, go faster
