Loops or a nest of thread underneath leather do not automatically mean the bobbin tension is wrong. The visible location of the mess is not necessarily the location of the fault. Start by identifying which thread forms the loops, when they appear, and whether the machine still turns freely. A jammed machine should be stopped and disconnected before you attempt to free the work.
Avoid yanking the project away from a trapped needle or packed hook area. Follow the manual's method for releasing a jam, and stop if parts remain locked or access is unclear. Cutting accessible loose thread may be part of that procedure, but pulling harder is not a substitute for it. Preserve the sample because the beginning of the tangle often contains the most useful evidence.
Decide whether the nest occurs only at the start
A tangle confined to the first few stitches suggests a different investigation from loops along the entire seam. Check the starting procedure in the manual, including thread tails, take-up position, and presser-foot position. Do not assume that a routine learned on another machine applies unchanged. An industrial lockstitch machine and a household machine may need different handling details.
Practice several separate starts on fresh scrap rather than one long line. If each start fails but the rest of each seam is clean, record that pattern. If the first start works and later starts fail after cutting thread, note the difference. These comparisons isolate the operation that changes instead of sending you directly to the bobbin-case adjustment screw.
Rethread the upper path completely
Remove the upper thread and start again using the model's diagram. Trace every guide and the take-up lever. Where the manual calls for threading with the presser foot raised, follow that instruction so the thread can seat correctly. Lower the foot for sewing. A missed guide or incorrectly seated thread can defeat attempts to improve the stitch with the tension dial.
Our threading guide offers a checklist, but the machine diagram controls the exact route. Check that the thread can leave its spool or cone properly and is not wrapped around a support. If it catches intermittently, a short successful test may hide the problem until the next section of thread pays out.
Look at the upper tension setting after rethreading. If it was changed repeatedly during the jam, return to a documented successful setting or the manufacturer's starting guidance. Make only one permitted change at a time. Changing several controls before testing prevents you from knowing whether rethreading solved the problem in the first place.
Inspect the bobbin arrangement without guessing
Confirm that the bobbin is the correct type and is inserted in the direction specified for the machine. Follow the required thread path through the case or holder. Check for obvious poor winding, overfilling, loose strands, or damage. A bobbin that looks roughly the same size as the correct one is not a safe basis for substitution.
Try a correctly wound known-good bobbin of the same suitable thread when available. Keep all other settings unchanged for that comparison. The bobbin thread guide explains why bobbin choice and thread choice should be documented separately. Do not mix in a different thread size and then attribute every change to the bobbin itself.
Clean accessible lint and loose thread using the manual's recommended procedure. Do not dismantle the hook assembly beyond routine owner maintenance merely because thread has collected nearby. A recent jam or needle strike can leave damage that deserves inspection. If the machine still binds after the accessible thread is removed, arrange service instead of forcing it through a cycle.
Use contrasting thread as a diagnostic aid
On a test sample, contrasting upper and bobbin colors can help reveal which thread appears on the wrong side. Keep the thread specification suitable and as comparable as possible; the purpose is visibility, not a change in performance. Photograph both sides and label which color was threaded through the needle. This makes a technician's remote assessment easier too.
Use leather tension adjustment to interpret the seam after the threading and bobbin checks are complete. A balanced-looking top can conceal loose loops below, so inspect both faces. Also distinguish a true loose loop from a long span caused by a skipped stitch; they require different lines of investigation.
Test the same assembly that failed
Include the project's lining, folded seam, and reinforcement in the next sample. An easy pair of offcuts is useful for establishing basic operation but does not prove the full assembly is ready. If bunching appears only at a crossing, observe material support and feeding there. The foot may be encountering a different situation from the one tested on the flat section.
Keep the project supported and avoid pulling it out of the stitch area before the machine has completed its normal stopping procedure. Removing work at an unsuitable point in the cycle can create confusing thread resistance. Follow the manual rather than repeatedly tugging until the thread gives way. A controlled start and controlled finish should both be part of the final verification.
Return to the project only after repeatable starts
Confirm several clean starts, a representative seam, and a clean finish on fresh material. Inspect the underside throughout, not just the final inch. Record the successful needle, thread, bobbin, and threading arrangement. If the nest returns despite correct setup, or if the hook area shows damage or abnormal resistance, request model-specific service with the failed samples and your observations.