The machine sounds normal, but a seam contains long gaps between otherwise neat stitches. Before tightening every dial, keep the failed sample. It can show whether the problem occurs everywhere, only across a thick join, or only on a particular fabric.
Start with the needle you can identify
Switch off the machine before replacing or handling its needle. Check the machine manual for the correct needle system, insertion direction and installation procedure. A needle can look straight and still be worn or unsuitable for the material. Coats’ needle guide explains why needle size and point selection need to match the sewing application.
Keep the original thread and fabric while testing a fresh, appropriate needle. For a knit, consult the needle manufacturer’s fabric recommendations rather than assuming the needle used for woven cotton will work equally well. Do not choose solely by the largest number on the packet.
Make the sample resemble the seam
Use the same number of fabric layers as the project. Include a folded intersection if that is where the missed stitches occur. Sew a short line at a moderate speed without pulling the fabric through. Mark the result, then change only one relevant factor before trying again.
A useful notebook entry might read: “Two layers of jersey; fresh recommended needle; same thread; skips only across the four-layer join.” That is more informative than “machine still broken.” Rethreading according to the manual is another sensible check, particularly after a thread jam.
If the issue persists across suitable needles and properly threaded test samples, stop before damaging the project. Keep the samples for a qualified service technician. Timing, mechanical damage and other machine faults are not problems to solve by forcing the fabric or randomly increasing tension.