I don't know if anyone else has had this problem and figured out like I did that it was indeed MY fault.
I discovered that when I am quilting, usually using my walking foot that I inadvertantly hit the tension knob when I am turning my quilts.
I had one that looked like this & I was very disappointed. Then noticed that my tension was WAY down. I have a sticky note on the front of my machine to remind me to check my tension frequently.
Happy Quilting, Susan