if a move doesnt get the tap for you, one solution is using it as a transition. you get an armbar and someone rolls and you end up both face down (this is what i think you mean) then just take their back.
go for the RNC and if that fails too, back to the armbar. he might not expect this and it will be easier to finish, plus the momentum is at a minimum and he should end up flatly on his back. again, easier to finish.
as for the armbar itself, if you're talking straight armbar, when you fall back put one handon the wrist and one on the inside of his elbow. if im armbarring the left arm, my left halds his wrist, pulling it near my left nipple and my right hand cupping the inside of the elbow keeping the arm bent slightly so i dont slip off and he cant roll.
then i bridge up and to my left, the way the joint goes. i usually keep my right hand cupping the elbow. it helps keep my hips tight.
