Do you know which model of reducer it is? I assume since you are using an ES ED102 that it's the SFFR102. If so, then it's both a reducer and flattener, and it should say it right on the barrel. It's designed for the f/7 Stellarvue SV102T to make them f/5.6, but also works with the 102 Access refractor too. I'd suspect it should work with the ES ED102 as well.
I don't see any Stellarvue reducers listed on their site that are not also flatteners. (Though there are flatteners that are not reducers.) So it should not be a field curvature issue.
As noted, I don't think you can install it backwards. But just to be sure, the side with the female threads is toward the scope, the male threads are toward the imager.
If you are getting odd shaped stars, it may be tilt, or sag from the added length and weight of the reducer. Also, did you verify your spacing from the reducer to your imaging sensor is 55mm? If that spacing isn't correct, performance will not be optimal.
Edited by Chuckwagon, 13 September 2019 - 12:37 AM.