Using the specified Bobbin for your sewing machine as listed in your user manual is important for your machine to sew correctly. 

Vintage machines have much more variable tolerance in size and shape of bobbin within the bobbin class size (15, 66, etc) and can use generic bobbins without much trouble.

New modern sewing machines use specific bobbins for each type of machine and will not work well with generic bobbins such as sold by JoAnn's or other online outlets as whatever size specified.   

Read your manual and follow the manufacturers guideline for what exact type bobbin will work in your sewing machine and give best performance.

Why does my Brother sewing machine bobbin keep wadding up?

Brother sewing machines take a special bobbin to work correctly - they are Bobbin# SA156 available for about $5.00/pkg of 8 bobbins at Walmart of Amazon and other places that carry Brother sewing machines. Your Brother sewing machine will work like a dream if you use the correct ones for your model.