Maybe just the controller settings or that it's not working to specs. I've gone round and round with customers who say their equipment bench tests good at the shop but fails in the field...until they come out into the field and a lot of times the word is "oh". Then again it might be something staring you in the face and you're missing it... been there done that
It took me about 3 times before I realized that my Espar was working right and I was operating it wrong. They kept sending it back saying it works fine on the bench. I finally realized that when it coded out and stopped, if I'd had just sat back drank a beer and let it cycle the damn thing would restart itself and run correctly. It's amazing how a few beers can fix things
For me it was one of those "oh" minutes