Oh! Silly me confession time: One time I spent quite a lot more time FMQ than it should have to realize that something wasn't right at all... turns out I forgot to put the foot down... there were some stitches and I was focused on what was ahead of me and I wasn't watching the disaster behind me.
As we are ruling out things, I always reset my foot to make sure it is on correctly, and then you have adjusted the pressure/height to work with your batting/fabric combination?