I had that problem the other day and I solved it adjusting the tension. Dealers don't recommed to adjust the bobbin tension but I had to because I couldn't just fix it by changing the needle tension.
If what you are showing is the back of your quilt then your needle thread tension is too loose (or your bobbin thread tension is too tight)
Before changing anything, saw a straight stich on the sandwich to see if the tension is fine, then continue trying with the darning foot and tensions.
Good luck!