lower tension incorrect?

I'm trying to replace a zipper in a heavy weight hoodie; tried #11 and #14 size needle with a heavier weight thread. Tension is set at 4 and have tried 4.5 and tried 5 yet I get a few stitches then a balling mess follow by a few descent stitch etc. underneath. the only good thing is it is easy to ripp. I have re-threaded and re-done the bobbin (using the bobbin and needles that came with the machine). Other suggestions would be greatly appreciated.