Mirror Skin Weights gives me the noodle arms, despite previously working

[Anonymous]

Hi there, I am a student in animation and I'm trying to figure out how to mirror my skin weights properly. 

In the past it worked seamlessly on this model, but now that I've finally gotten the arms the way I want to, I can't mirror the weights anymore. Is there a reason it might fail? I've tried everything I could think of, removing the bad side from the model (so when I mirror it's pasting ontop of 0 influence), bringing the character back into bind pose... 

I'm sort of at a loss, and I don't really know if it's write to blame that on a bug. I  must be doing something wrong... :< Any help or suggestions of what I might be missing would be super helpful!