This is presuming that you arent seeing bb's in the top of the mag... (if you are, is it a different make to the others as not all mags have the same top profile, so not all mags work with different makes of guns)
Firstly, did you wind it till it made that wierd noise that tells you it is fully compressed? (if not then your winder may have detatched from the spring)
Secondly, have you tried a littel physical "persuasion", i have a cheap M4 mag that came with a gun and it tends to jam up at least once a day, repeated blows to various areas will cause it to sound like it has just wound out and presto, it works again.
Hope that helps.