The metal bobbins Sew-Classic sells are fine with one proviso. The little notch that engages the little tab to spin the bobbin is sometimes not big enough. The plastic ones are. However I have some of the new bobbins from S-C and do use them in my 15-91 with good success.
The bobbins from the box stores are hit or miss in quality. Before declaring the bobbin case at fault, try some different bobbins.
Also remember when you put a full bobbin in the case it should turn clockwise when you pull on the thread.
And it doesn't hurt to check the tension on the case. Just because it's new does not mean it's correctly adjusted.
Joe