I went back to your original post and you don't say whether the pre-wounds are on a 15J bobbin?? Different machines, different bobbins, even though a lot of them look alike. I know my Viking will only take Viking bobbins so I haven't been tempted to try the pre-wound.
Were the pre-wound a better buy or, are they just convenient? (Nothing wrong with convenience, that's for sure!) _:-)_