Quoted from mrm_4:but for some reason the back won’t cure. It sits in my garage next to a window and the sunlight comes in the window directly on the back.
One thing I always make sure to do is make sure my paint and my work are the same temp. If I'm painting outdoors, I leave my paint outdoors (in a shaded area) to reach the same temperature as my work, same if I'm painting in my booth.
I've had similar issues to yours if I don't remember to do this.
If you back had been in full sun, even in OH, perhaps that caused the issues?