a few weeks back i was have a tension issue i had tried EVERYTHING i knew to fix it, NOTHING but frustration.... prayed for a miracle finally took it to the shop & I had NO IDEA about this part of the tension & perhaps u did this already but sharing JUST IN CASE.... on the bobbin casing is a screw this screw too loose can make the bottom stitches too loose like mine was the repairman fixed it right up, works perfect now, then a few days later i find this page on the net.... perhaps this isnt ur problem but i know many others r as unaware of this area for tension.... good luck
http://www.threadsmagazine.com/item/...nsion/page/all