My 15-91 is my all time favorite machine, but I have experienced the same as you with some of my bobbins. I think my problem is the groove in the bobbin is too shallow to stay engaged. If you can find vintage bobbins, that would most likely solve the problem. Sew Classic now advises using their plastic bobbins for the 15-91. I have some, they work fine, but it almost feels wrong somehow to put a plastic bobbin in my beautiful vintage machine... I know, l'm nuts! There's just something about a nice, metal vintage bobbin...
Joe mentioned a great solution to reduce frustration - when you find a bobbin that works, keep it with that machine. I think I will start sorting which ones work and find a different use for the defective ones...