I remember last year at retreat the lady sitting next to me was having lots of problems (can't remember the make of machine) She wasn't well so went to lay down. I asked others who knew her if she would be offended if I took a look at it. They said she would probably thank me, so I undid her presser foot and removed the bobbin, cleaned it out but there was still a problem so went back in and there was a piece of cotton about 3 inches long. This is what was causing her problems and the machine had not long been serviced. Machine can be very temperamental these days. As DH says the more bells & whistles a thing has the more can go wrong. Still wouldn't part with my Janomes.
BTW She was very grateful and gave me a big kiss on the cheek.