Play with the bobbin tension...I do the drop test and with my tension I have to kinda shake the string before the bobbin starts to drop...it works for me...it is just all in re-adjusting tension and testing until it is just right....also it depends on the weight of the thread...are both top and bottom the same thread weight or different..cotton or poly it all makes a difference in tension settings....Try watching this video
http://www.youtube.com/watch?v=OTmT-...eature=related