Could be any number of reasons: a corrupted PZ3, Vue read it wrong for some reason, a MAT file (if there is one), unusual object groupings. I've had it happen before, but not since patching to (I think) Vue 4.03. Even before patching, usually closing Vue and trying again would work, after making sure, with Poser that there wasn't something wrong with the PZ3 file, and saving it again. I'll have a look at it and see what happens.