I'll keep you posted. I've contacted Eternabond for advice but haven't yet heard back.
The news about Acetone not being a complete cleaner was news to me, and is a possible factor. But I cleaned the areas 4 times. Maybe it didn't remove All the dirt, but surely this can't explain a complete 100% failure? Still, Eternabond recommends Acetone as a cleaner and makes no mention that other cleaners must be used as well (although they do recommend their own Eterna-Clean foremost).
I did say it was a cold day and there was frost on the roof, which I removed and thoroughly dried, before cleaning with acetone. I guess it is possible that the air was humid enough - and the roof surface cold enough - to cause moisture to re-condense before applying the tape. But I didn't feel any such moisture. And as I said, it sure seemed well-stuck right after finishing the job, and a day layer when I put the cover on for the winter.
Defective tape is a possibility but very unlikely. This was bought last year. We'll see what the company says.