WIthout actually seeing you making the mistake, I'm just going to assume. I'm sure others can help better. I've been having some success with the electric chair and the more I practice it, the more I have to trouble shoot.
My guess? You're not deep enough. For the set up when you whip up, get to your side. One arm behind hugging the far hip, one arm under the thigh, head tucked deep either on the stomach or preferrably on the thigh being underhooked. When you fail old school (they post their leg) and you angry rainbow your arm that's behind him should "heisman" stiff arm him far away from you then get up to your elbow. There's no way he should americana you from there.