Sounds like a shaft may have bent or tweaked out of alignment. I owned my own Upholstery business for over 20 something years before I retired from it. The most reliable industrial machine I ever owned was a Phaff 545, I had i converted to use Singer feet because of the price of the Phaff feet. It had a walking foot and reverse. My oldest daughter now uses it in her business.
I hope this Peter guy works with you and solves the issue. One would think after 3 times he would offer you a replacement. Not just a "deal" on another machine. BTW a Juki with a walking foot is a good machine.