Translucence works on IPR but not render

[bentway23]

Got a weird problem--I have a simple lampshade with just a little translucence and a point light behind it. The translucence isn't working as it should on either frame/render view render or batch render--but it works perfectly in IPR. Any suggestions? I'm using Maya 2016 and rendering using mentalray. I've tried the scene on multiple computers, thrown out preferences, &c., &c.

Thanks!