Adding tapes to DR4100 Virtual Tape Library with DPM 2012R2

Hi, I've been using a Dell DR4100 for iscsi VTL with DPM 2012 R2. I followed the whitepaper "Setting Up the Dell™ DR Series System as a VTL Backup Target on Microsoft® Data Protection Manager" here to setup my DR4100 and DPM Server with 20 tapes. I have been using this successfully for a few months now but have begun to run out of free tapes.

I added 20 new tapes via the DR4100 interface follwing instructions on the document mentioned above. This leaves me with 40 slots on my VTL, but the new twenty slots are showing empty. The question I have is, is this expected behaviour, or is there something else I need to do in DPM to see tapes in the slots? 

I have one free tape at present in the original 20 slots, and perhaps once that is used a new tape is created automatically? Of this I am not sure?