margaret,
Up until about 7 months ago I thought sewing machines were some sort of black a magic, voodoo, machine. Then I actually opened one up ( my first 66-4 treadle machine ) and found out they are about as simple a machine as a human can design. They have their quirks, but basically they can only work one way. (Well the old ones any way.) About the only things I can think of that will stop them up is rust, gummed up oil and grease, lint and debris, and operator miss-assembly. Wear won't usually stop them, it just makes them noisy and throws the adjustments out. That's where the oil and grease comes in
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Joe