Stitches Looping Under The Fabric

1. Your machine may be not threaded correctly. Re-thread both top & bobbin thread.
2. Your needle may be damaged, or just secured not well enough.
3. Your top thread tension might be too loose. To check whether the problem really is in tension, remove your hoop and inspect the reverse side of your embroidery. If your tension is well balanced, you will almost not see the top thread on reverse side.

However, if you see a lot of top thread - increase top thread tension. The loops may occur just because the needle is catching on those bubbling threads.