Yesterday, my friend's daughter, Fluffy, visited.
She needed help with her sewing machine, which quite frankly, after looking at it, I thought it could do with binning. Every time an attempt was made to sew the thread jumped from the tension disc, which was very difficult to alter, and the whole machine was sealed, so you can't actually take it apart to service it. The only thing I can think of that is causing the problem is the feed dog is full of lint, and that could be removed by vacuuming it.
Anyway, we tried my old machine, and that didn't want to play ball either. So I used my newer machine, which gave no trouble at all.
(I have since suggested to Fluffy that she tries vacuuming out her machine, that's about all she can do.)