Have you installed the latest HIT kit, v4.7?    It sounds like you are running EQL fw v7.x?   A change in snapshot behaviour was introduced at customer request.  If a snapshot is online, don't delete it w/o first putting it offline.  HIT v4.7 has code to do this when a VSS delete snapshot is requested. 
Hi, I've got a three nodes Hyper-V 2012 cluster connected to an iSCSI SAN, a PS6210XV. Currently I'm backing up the vms using Microsoft DPM 2012, using EQL hw provider. The backups are running fine and fast, but every time the DPM runs a backup, the PS generates a SAN snapshot of the volume containing the vm. When the backup completes however, the snapshot is not deleted, but remains online until someone deletes it.  How can I have my snapshots automatically deleted after a backup job? I don't want to use the Hyper-V software provider to do my backups. Thank you :) .
Delete PS snapshots automatically - How to?