I have used very heavy quilting thread in the bobbin and invisable thread in the top with great results. Needle size , plays a BIG issue( the smaller the better) , as well as tension, and good manufacture of the thread. Do not forget about how the thread is coming off the spool , for best results use a cone holder or cup. Sometimes you can make all the tension adjustments ... but of the thread is too tight prior to the tension disks.. sometimes all the loosening is still not going to help.