WOW! I just fixed it! It was the reverse button. I knew it was gummed up and stuck, but thought I had it pulled out to the full forward position. Apparently, it was still pushed in just enough to keep the dogs from biting. I shot the whole area where the button pushes inside the machine with one of my favorite products, PB Blaster, a penetrating oil, and force worked the button in and out for several minutes until it loosened up and self-retracted. When it did, I heard one little click that I hadn't heard before. Voila!
Thanks for the encouragement. I think I would have given up without it.