30 yards of fabric? IMO they certainly should have been able to figure out your problem by now.
Do you have Skype? Perhaps you can do a Skype call so you can actually show them what is happening in real time.
I'd certainly be unhappy if it wasn't working after all this time.
As for the tension, Jamie Wallen has a video on how to check the tension of your bobbin that people refer to all the time. Maybe have a look at that first off?
And no, it isn't time to go back to hand quilting, it's time for Nolting to get their act together and offer you some support to help you fix this.
Watson