If you are working with ab bigger Operations Manager environment, then you perhaps already realized that the objects shown in reports are limited to 500. Also if you select more objects, the report will only show the first 500 objects.
This problem already existed in SCOM 2007 SP1/R2 and still exists in SCOM 2012.
A solution which is working for both versions is described in this knowledge article: http://support.microsoft.com/kb/963005. In SCOM 2007R2/2012 you should not install the hotfix, but you can create the registry key as described in the chapter “Post-installation instructions”.