Invisible Jiu Jitsu teaches and tours enough to see how often the traditional armbar from closed guard fails some grapplers. Part of it is because most belts, regardless of where they train, learned the OG closed guard armbar the same way and feel it coming. The other part is user error which is easily corrected.
To upgrade your submission attempt David Morcegau suggests breaking your opponent’s posture first. Use that opportunity to get a good grip on the back of their collar, which prevents your opponent from slithering out of your reach. Cupping their elbow, open your guard on the same side and bring your heel to their hip to push the cupped arm through the center line of the body with your thigh. Finally you’ll swing your opened leg over their head while “pen-knifing” (getting your butt off the ground to post on your shoulders) and raise your hips to hyperextend their arm.
He also reviews what to do if the armdrag that could set up the submission goes a little too far: