I'm not sure but maybe this will help. Try threading the machine over again. Sometimes something just doesn't fall in the correct spot. Try another bobbing, or change the needle. Or adjust the tension. I have the 6600 and when the bottom looks loopy iI rethread the machine and that seems to take care of it. It only happens once in a great while. Hope it works for you.
Thanks, I tried what you suggested, a couple of times and still all LOOPY on back of fabric.
Someone told me it could be an interior problem. Last resort will be to find a repair shop close by...
Have good day
Is this a new problem with a machine you have had for some time or have you recently bought this machine? If you have had the machine for some time try this. Remove the bobbin and unscrew the plate under the needle, around the feed dogs. Then carefully remove the bobbin case while checking how it was seated (you will need to put it back). Take a soft,small brush ( eyeliner brush is good or you may have one with the machine) and thoroughly clean the cavity and also the bobbin case. replace and rethread.
Be sure the thread is running in the correct direction off the bobbin. Draw the bobbin thread up and hold out of the way when you start sewing. I had a serious problems with loops when using a narrow hem foot. After trying everything under the sun for an hour. I finally tried cleaning the bobbin area. Et voila, problem solved!!! It didn't seem to have much lint in it but cleaning worked and made me realize that these are precision machines we are dealing with. Also refer to your manual and check your bobbin tension. Good luck.
Ladysmith BC,Janome 12000, Bernina Record, Bernina 1150MDA serger, bit of a Luddite, garments not crafts