Turns out that my cutting mats were so worn that I'd created grooves in certain places and when the rotary cutter hit those, it didn't cut like it should. My mats are reversible, so I flipped them over. A few months ago I taped the underside of my two mats together to make one big cutting mat for cutting long lengths of fabric. I used duct tape. That's sticky stuff and when I flipped them I took off the tape and of course left a line of sticky goo. What to do?
Well, I googled for a solution and found this site. I looked over the list and when I came to the 3rd item I knew that a tube of toothpaste was just steps away. So, Crest to the rescue.
One little line of toothpaste, a bit of rubbing, and a quick wipe with a damp paper towel and I was back in business.
The magic of toothpaste...and my mats are minty fresh too.