Also make sure needle is in correctly.
I just added to my list of things to consider, spray baste.
I started a project yesterday that I spray basted (normally I hand baste my quilts) that had sections with loops and tension problems. When doing test runs on same materials just minus the spray baste, the stitching was beautiful, but when I went back to the quilt with the spray baste I had sections of loops.
This may not be your situation, but just something to know about and consider when looking for solutions to machine problems.