Could the fabric cause this?
 

When I was quilting this my machine would occaisionally throw a "knot" on the back of the quilt. Then when I was sewing the binding I experienced the same thing. I used two different machines. My Janome 6500 does the quilting on a frame and my Baby Lock was used for sewing the binding. Now I'm wondering if the fabric is the cause of this. The fabric is kind of stiff and definitely top quality.
Any thoughts?
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Tried to edit to say the fabric is NOT top quality.

It could be bumps in the fabric that is putting off the stitching but mostly nests like that are a needle/tension/thread/threading issue.

mgmsrk - I agree, but it happened on two different machines. That is puzzling me.

Could be the thread. What are you using? Are you using the same in the bobbin?

Some thoughts would be clean the bobbin area and make a practice sandwich with the same fabrics and batting, and practice fiddling with tension and etc.. see if that helps...

I get this with my Pfaff when there is a build-up of lint in the bobbin case.

When this happens to me, I take my machine apart and clean it. Like Joyce said, it is usually lint build up in the bobbin compartment.

Same here....it's amazing how much lint and stuff builds up. I have a stiff little paintbrush and a sate stick handy to keep everything ticking along.

could be time for a new needle- along with that recommended cleaning...imperfect stitches are often caused by a dull/old needle- and lint build up in the bobbin area.