Some of us buy bobbin cases without fully considering they might only work for metal bobbins.
The plastic bobbins 'fit' in okay, but they still can roll around, causing thread to get loose. Additionally, they can fall out of the case extremely easily. This isn't really helpful.
The fix will cost you only pennies. And you can always get your money back.
Inserting a penny next to each bobbin stabilizes them and keeps them from rolling around.
Of course, you could always cut out circles of craft foam and use that instead, but pennies are easier, and the perfect shape and width.