I'm no expert on machines, but I'm not sure the fleece is the problem. Unless you tried sewing on something else and the tension was fine? When I'm having a problem like you describe I go down the list, sewing a line of stitches on a scrap at least similar to my project to check if I have solved the problem. My list includes: rethreading top and bobbin threads, changing the needle, changing the bobbin, trying a different thread, cleaning out the bobbin area. Hope you find the problem, Humpty Dumpty's cold!