Re: Kotegaishi weakness?
When you do irimi senkai turn your body into uke's elbow. If you do it right you will cause enough pain to relax the wrist. Also when uke's balance is broken (he only has one foot on the ground) quickly turn back and apply the throw. Its almost impossible for anyone to resist kotegaeshi if they are off balance. Also doing it this way makes it very difficult for uke to strike you when you apply the throw.
Another method, if uke has a clenched fist try rolling his fist towards him. This also helps when uke doesn't leave his hand out after punching.
I think with most techniques it does help to train with someone who is not afraid of being thrown or has no idea what you are going to do. You probably noticed a technique works fine the first time or so on someone but then becomes difficult.