If you call another (nested) mzscript bang before that, it will wipe out the parameters for the current one. Only thing I can think of... Unless you've got a !VarSet \2 in there someplace, but that'd be too strange and obvious.

Did you try an !alert in exactly the same place in the script? Or better yet, use !alert '!LabelMove Fish %{\2} %{\3}' ?