I purchased a package of class 15 metal bobbins at Walmart a few months ago. Most of them were ok. One has to placed on the winder on one side only. The other side only goes on so far and sticks. I see no obstructions and have no idea why the bobbin sticks.
I am so happy that my mil made the mistake of purchasing 4 class 66 bobbins many years ago. They works well in my redeye treadle.