Magtape deallocation will not work properly in VMS/IRAF if the CL is run from a privileged account such as one that has BYPASS or SYSPRV privilege (a cl> !dismount may unmount the drive).
Under VMS 5, the AUTOGEN procedure includes feedback information. It is worth running the system, with IRAF users, for a couple of weeks and then re-running AUTOGEN (as detailed in the VMS System Management documentation) to adjust some of the system parameters (global pages, various memory structures) for the new work load.
"If all else fails", e.g., hung tape drives, etc., -- our VMS system manager recommends kicking everyone off the system, perhaps even rebooting if it gets desperate, and then inspecting this Installation Guide again, before calling us. And please, if you need to call, try to have available the exact text of any error messages you may have encountered.