What should I do if the upper thread tension is too loose?
If the upper thread tension is too loose, loops will appear on the back of the fabric. Increase the tension by turning the upper tension control dial to a higher number. Also verify that the upper thread was correctly threaded, as incorrect upper threading can cause the thread to appear too loose.
From the official Brother XH2149-001 Computerized Sewing Machine manual, page 1
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