You’re right, an hour and ten minutes is an eternity to wait for an answer on a forum (from your first post to your last), especially at that hour.
In all seriousness though, I’d be looking at your repeat backing plate failures as a symptom, not as your problem. Plenty of professional detailers are using these machines without having to feed them backing plates all the time.
Tell us more about your use. Is it both your 15mm and 21mm? What speed are you running them at? What pads are you using? How many pads are you using per step? How much downward pressure?
Any time I’ve read about backing plate failures, one of the issues could be prolonged running at too high of a speed. There’s huge accelerations taking place at the pad face on a long throw, and each time it changes direction that’s a lot of force acting on the Velcro. Any small movements there create friction —> heat —> plate (or pad) delamination.
What pads? If not Rupes pads, I’d wonder if the pad you’re using uses a slightly different hook and loop fastener. Again, leading to more movement between the pad and backing plate heating it up.
How many pads? Here too - if you’re using a pad for too long and it’s loading up with product, you’re increasing it’s weight and it’s resistance to changing direction all those times per cycle. Back to more heat, more failure. Are you also running into doming of your pads, where the center starts to cave in? That’s another sign of too much heat.
Just points to ponder.