they call that 'railroad tracks' when the bottom thread just lays there. It means one of two things: but don't adjust both at the same time. Either your top tension is too loose or your bobbin tension is too tight. I would try the top tension 1/4 turn to the right (while chanting 'righty tighty lefty loosey'...sorry! I could not resist!). Adjust the bobbin tension SPARINGLY, especially if you had good stitch quality just sewing. If it is the other way around, and you are getting pokeys on the TOP, the top tension is too tight, or the bobbin tension is too loose.
Hope this helps and is not too confusing..