Cover art not showing for some movie collections

Hello all,

I have a bit of a strange issue.

First, my system:
movies stored in 2 different folders on the same "Server" (actually a windows 7 PC that is always on).
mysql library
sources and favorites xml files stored on the NAS
sources.xml references the NAS username and password like this:


all xbmc installs are frodo and they all use the exact same advancesettings.xml file pointing them at the sources.xml on the NAS

The issue:
Any movie collections which have movies on both of the movie folders will not show cover art for the set. Selecting the set shows cover art for all the individual movies though.
Any movie collections which have movies on ONLY ONE of the movie folders work perfect.

This happens on all my devices (ipad, windows pc, raspberry Pi). The only device that shows all movie poster are properly is the "server" PC on which the movies are stored. Even though it uses the same advancedsettings.xml files as all the other installs.