Moen Single Handle Shower/Tub valve problem

[short.circuit]

I installed this valve on a friends tub/shower in her basement.

http://www.moen.com/shared/pdf/82910pt.pdf

when we turn the hot water on, it comes out the spout just fine.
if we turn some cold on with it, some water comes out of the shower.
should we bug moen, about getting a replacement cartridge? or would that help?
thanks in advance for any advice...

here's a pic of the install...