One consideration:
When you have a bike that is perfectly comfortable outside, and then leads to problems when ridden inside, I feel it is important to look at what is different. When you do, there are two key differences.
- Lack of wind resistance on the body riding inside. That is a difference that I find because you end up with slightly more weight on the hands and arms, because you don’t have the wind pushing your upper body back.
- To compensate for that, I recommend that people raise the front axle about 1"-2" [25mm-50mm] higher than the rear axle. This shifts the weight slightly back onto the saddle and off the hands and arms.
From: Rocker Plates for Trainers
As suspected, I think a rocker plate could help, especially if you think the locked-in nature of the trainer is the issue.
I would suggest testing the higher front wheel option shown above, because it is quick, easy and cheap.
If that doesn’t fix your issue, consider a rocker. There are some quick ways to hack one for another test, but they are more work than the front wheel trick.