When was the last time the check spring was replaced? This is the spring that is in the top tension assembly. Wondering if the dealer bothered to replace it when selling it used and even if they did, these springs need to be replaced on a fairly regular basis, it all depends how much use the machine is getting. And it is not out of the realm of possibility you have a defective check spring even if it was replaced with the dealer service before selling. I know of some LAers who replace them every 6 months and others who can go a couple of years. The loop of top thread on the bottom like you describe and picture is indicitive of the check spring needing replacing. Aside from that one area your stitches look great.
Other things that can affect your tension mid bobbin:
an uneven wind on the bobbin where tension has changed during the winding process. This can happen on prewounds as well as ones you wind yourself. I use magnaglide prewounds and I got rid of a lot of tension issues with this product. But even with magnaglides my tension varies between a completely full bobbin and one that is 3/4 used.
A blob of lint sloughing off and getting caught in your tension spring. This is a big reason the overwhelming majority of my quilting is polyester threads. Cotton, no matter how good will make a lot of lint with the high speeds these machines run at. Also you get lint from your backing fabric. I always notice a lot more lint when I have a flannel back quilt.
Quilt loaded too tight on the frame (you didn't say if this is table mount or rack mount so if this is a table mount machine scratch this suggestion)
Edited to add, you machine may be just a teeny tiny bit off in timing. Ask your dealer what needle size they timed it too. Usually timing is set for anything from size 14 to 18 but if you switch up to those much bigger sizes you might have issues. I usually quilt with a size 14 or 16 depending on what thread I am using and don't have to retime my machine.
Last edited by feline fanatic; 08-18-2019 at 08:58 AM.