Let me be the first to give you some bad advice
Hit him... Not eye popping out of the head hard, but enough contact that uke notices deliberate atemi. Ushiro works great when you atemi to the ribs as you step under uke's arm. Let the clod learn that when uke latches down hard on one arm, which is only roughly one ninth of the body surface, that the other eight ninths are then at complete liberty to smack him at will. It is very easy to stop an aikido technique when you know what technique is coming. You owe it to uke to move on (atemi or different technique) if they are deliberately thwarting you.