Why this sewing machine problem?

http://www.picturetrail.com/sfx/album/view/2252155... shows what happens with straight stitch (not running stitch, which is a hand stitch) with various common misthreadings. The photos are in pairs, showing the top and bottom of the fabric. You've probably threaded with the presser foot down, or the top tension is loose or you're not starting your seam correctly and remembering to hang on to the thread ends for the first couple of stitches.

First of all it is called a straight stitch.

Secondly, you probably threaded the machine with the presser foot down. This closes the tension control and the thread cannot seat into this device and when you begin sewing, the tension setting has no control of the flow of the thread to the needle.

What appears to be a problem with the bobbin is actually the top thead being loosey goosey and not holding the formation of the stitch.

First try removing both the top and bottom threads, then rethreading the machine. This will often solve the problem. Test on a scrap of material.

If the problem persists, then you may need to tighten the bobbin tension -- this is usually done by turning a tiny little screw on the bobbin case-- or loosen the top tension by turning the large dial on the front of the machine to a higher number. See your sewing machine manual for more info on changing the tension.

I think it's called the 'tension' setting--probably too loose.

You'll probably have to play w/ it (tighten in increments) to get it just right.