When I sew, it seems that the threads underneath are not being connected. So, there will be no sewing on the top of the fabric, but underneath, I'll see a bunch of loops, but nothing connected to the threads on top (does this in the regular serging mode and also using the rolled hem mode). I've replaced the needles, tightened and loosened the pressure foot, rethreaded everything a zillion times...I'm at a loss. Please help!!