Driving me crazy!
 

I had a problem with points not meeting on a quilt top, and someone on the board suggested couching yarn with a zigzag stitch over it to hide the problem. Great idea! It worked too! But all of a sudden my machine starts not zigzagging correctly. I get very loose threads, and even broken ones, which I really can't figure out because the thread continues to stay whole as it stitches. What's with that? How can it be?
I cleaned out all lint, I rethreaded, I tried changes in tension. I tested on a separate test sandwich and it works perfectly, until I sew it on the quilt sandwich, and then it messes up.
I've included 3 pictures. One, with blue yarn, where it's working correctly, and 2 with brown yarn where it messes up. The yarns, thread, etc. are all the same.
Any suggestions? Help!

Are you using a couching/cording foot?

Interesting technique. The two suggestion are what I would have said. I would be very careful with yarn since it is such a fuzzy item.

I am using a couching foot. When I try it on a test sample, it works fine with the yarn.

If it does ok on the blue, but only has problems with the brown, I doubt it would be a machine problem. I notice in your photos that the problem seems to occur at the intersections of four fabrics. Also, the bobbin thread isn't catching. I'd try changing the needle. A sharper needle might be able to go through all those layers easier and catch the bobbin thread.

I changed the needle, and it seemed to help slightly. I played with the tension, and putting it up very high seemed to make it work (although it's pulling the bobbin thread, which is a different color, up to the top more, but I think I can live with that if I can just not have those thread breakages and loops).

Are you sewing on different color fabric when you are having the problem? Are you doing your test swatch on the same fabric as your quilt? I had a similar problem and turned out to be the fabric. I was using 4 different batiks that were purchased at the same time from the same shop. I had no problems with three of them but the one blue I got skipped stitches. It was driving me crazy too. I cleaned the machine, changed needle, you name it I did it. I finally adjusted the tension until I got it stitching right and that did the trick. Different fabrics and even different color fabrics react differently.

The problem is I have hundreds of different fabrics on this scrappy. I think it must be the number of layers that I'm going through--I can't really reproduce that on the test samples, and that's the main difference. I also think that it is the tension that will govern ultimately. I just have to figure out what to do about all the messed up ones--I guess I'll have to pull them out.
Thanks!