Is there an online guide to sewing machine repair and problems?
My first suggestion is a good, thorough cleaning and oiling. Use a vacuum, not canned air, to get dust out, and pull the needleplate to get good access to the bobbin area. Oil per the manual’s recommendations using genuine sewing machine oil, not 3-in-1 type household oils nor WD-40 (it’s a solvent, not a lubricant.) If these machines are modern (last 10-20 years)non-mechanicals, and they have not had a COA (clean-oil-adjust), they’re probably more than overdue. If they’re mechanical machines, you can learn a lot about doing a basic COA by joining the yahoo group “wefixit”. Electronic and computerized machines probably need to go to the sewing machine mechanic for a COA. At any rate, first try the cleaning and oiling regimen found in the owners manual, and then put in a new needle and rethread with manual in hand. Make sure you’re using the right bobbin — visually, there’s not much difference between a class 15 and a class 66 bobbin, but the machine will pitch a fit with the wrong bob
