Unfortunately, you just discovered one of the major shortcomings of Modern Aikido training: The "ukemi" model. You are trained to give up your center and fall in a specific manner. So, everything is going to work fairly well in the dojo. Everyone gets trained to do this.
If anyone disagrees, then try this and report back:
1. Get a teen wrestler with no aikido experience and try kotegaeshi, allowing the teen to use wrestling experience.
2. Get a BJJ practitioner and do the same.
3. Get a Judo practitioner and do the same.
4. Get a Karate/TKD practitioner and do the same.
5. Get an average Joe/Jane off the street and tell them to try not to comply and try it.
6. Get a Boxer and do the same.
Did any of them fall like a "normal" aikido practitioner? Or did a lot of other things happen?
Now, before you start in on all the rationalizations and excuses and explanations, answer the question on why Ueshiba could get 1-6 to work just fine, why everyone who trained with Ueshiba had to learn their own way of "ukemi" to protect themselves (they weren't taught by Ueshiba to roll and fall), and why this was also the case from the mid-late 1920s on ...
The modern "ukemi" model in Modern Aikido is a big problem.
Kotegaeshi was not working well against his strength but I did get a sankyo from a collar grab that had him spinning like mad on the floor. He's a trained wrestler and has some BJJ training; he actually tried to spin in for an arm bar on me. We did this in my living room so there wasn't enough space to pin him properly but I think I had him; and he admitted that he liked sankyo.
Just a disclaimer: were two very old friends and have been doing this stuff (wresling/goofing around) for many many years. We both know the risks and were just having fun.
Interesting comments to all of the above: I've learned quite a bit, thank you.