Fleece has a bit of a stretch to it, while your other fabric does not. It could be that you’re slightly stretching the fleece while sewing without realizing it and this is putting some strain on the cotton/poly, especially around the curvy parts. If you have a walking foot, you should consider using it for this as it should allow the fabric to feed through more easily. Or it could also mean that your thread tension is too tight and you may want to loosen it a little. For sewing with curves, it really takes a lot of practice to get good at it. For me, I take it slow, do a few stitches, put the needle down in the fabric, lift the foot, adjust, and then put the foot down and do a couple more stitches. I hope this helps.