Like CKCOWL said, the walking foot is not a magic cure-all. It can only work like it is designed to, on a properly prepared base.
I have one, but am not impressed with it. I can do just as well with regular foot.
You might find your foot pressure is too heavy for the thickness of a quilt sandwich.
Longer stitches tend to look neater, too.
Check your bobbin, again. Sounds like the bobbin tension is not correct.