Bobbin problems...
 

What causes the thread to come out of the"slot" while sewing...causing looped stitches on the bottom? This is a Pfaff 2170. Thank you for any help.

Some questions---Is this a new machine? Has the machine been sewing properly? Have you rethreaded the upper and bobbin threads? Is this a material you don't normally sew on? Is this a new thread type you are trying?

Now...info I have gleaned after a number of years of sewing. Looped stitches on the bottom typically mean that the upper tension is bad. Have you checked to see that your thread is going through the upper tension properly? There are other reasons for the problem. One that I had experience with was a tiny bit of thread somewhere in the thread path. Another way I caused that problem with an older machine I had, was by not lowering the presser foot. Luckily, my newer machines won't sew if the presser foot is up.

It's an old Mach e and I did the things you suggested and did a general cleaning and it seems to be sewing fine now. Although this was a sporadic situation...will see what happens��

I have that problem with my Singer 301A if I use Aurifil thread. It seems to slide out of the bobbin tension spring. Other threads are no problem.

when that has happened to me I clean out the bobbin case, take it out run a q tip all over and you will be amazed at the dirt

Perhaps those bobbin things will help if it happens again.