Snapshot transfers fail with errors related to the repository space and EPOCHS not being accessible due to "Delete_on_Close" Errors when the core tries to add a snapshot transfer to the agents current recovery point chain in the repository.
Errors you may see when this issue is causing the snapshot transfer errors:
ERROR 05/06/15 09:05:05  - Invalid header in 'F:\tevorepository\tuc
\87253ae1-c6e5-11e1-833f-00155d089b00DeleteOnClose_20289_20962.ReplayEPS' The parameter is incorrect
ERROR 05/06/15 09:05:05 The repository on the Replay Core has insufficient space to store the snapshot of volume 'E:'. Remaining protected volumes will be snapped if sufficient space exists.
There are 2 options available to get these recovery points / epochs removed from the repository so that there is adequate space for other repo based jobs to run and complete:
1) Simply reboot the core which will flush the buffers and dismount any files that may be still open taking up space
2) If the core server cannot be rebooted due to the backup server having other production based roles then you can stop and start the core service which will also purge the Delete_on_Close epochs
On the core server
1) Open Services and locate the Replay Core Service
2) Right click the Replay Core Service and choose the Stop option
3) Once the service has stopped close the Replay Admin Console
Start the core service back up and open the console and you should see additional repository space available that was previously showing as USED and snapshots and other core based jobs should continue without issue.