I was sewing fleece and fatt batt together to birth a quilt and naturally something was going to get into the machine. Well, I had a pillow sham I wanted to sew with the fleece then I would clean out the dust. NO!!!!! I was sewing cotton to fleece and it sewed fine, nice stitches, but when it got to cotton and cotton, irregular stitches. First thought, I broke it, second thought: time to clean! guess what? it works fine now!
Always clean your machine after using batting, fatt batt or not!