I've had a lot of problems with RSM mounts also. I have had the most success since uninstalling RSM Server and Host Agents, upgrading to RSM 2.0, and installing 2.0.2 patch for the host agent (the patch adds some additional hardware scan loops to give a better chance for success in finding the new volume before attempting to mount it). My hunch is this will fix the problem.
There used to be a restriction about mounting a snapshot to the same host the parent volume is attached to. I don't know if that restriction still exists, though.
I'm not sure if you use SecurePath, but it was causing problems until I installed SecurePathv40c_SP2.
It usually works ok now, but still fails about 10% of the time on "DeleteHostVolume". For some reason that seems more prone to failure than the commands to creat and mount the snapshot.
I've also had problems calling the RSM script to delete all the snapshots from a Data Protector post-exec command via the RSM CLUI command in a batch file. I use windows scheduler via similar batch file to create the snap and it works fine.