After an upgrade from DLP 14.6 to 15.1 the ability to perform detection and monitoring on Microsoft RMS protected files in DIM Environment stops working.
After our upgrade to 15.1 we went thru the steps outlined in https://support.symantec.com/en_US/article.HOWTO125749.html ("Prepare Symantec DLP and Microsoft RMS environment for monitoring") again and it should start working.