Last week I bought a job lot of 32 bobbins from a charity shop knowing that they might fit my Singer 15K machines. I knew they were the right type of bobbin, and I knew I would be lucky if half of them fitted. Here is the first problem. Some won't even fit onto the spindle of the bobbin winder.
The orange thread was already on the bobbin when I got it. All the bobbins were wound with rubbish synthetic thread.
Others will go on, but then the tab of the bobbin winder gets jammed between the two sides of the bobbin so the winder can't turn.
These are cheap and nasty modern bobbins. You can recognise them fairly easily. They have a join along the central tube, whereas vintage bobbins have a solid metal tube.
It is very irritating when you buy them from a sewing shop and end up having to take them back. It is even worse getting duds through the post from a supplier. However I was glad to get a load from a charity shop and pay over the odds and know the money was going to a good cause. I might even get 15 good bobbins out of this little lot.