Auto-delete old snapshots

We have Veeam doing vm backups, which generates volume snapshots on our SAN. The issue is, even though "delete oldest snapshot" is checked on the volume's snapshot policy, and "borrow space as needed" is unchecked, eventually the snapshot space fills up and the volume is taken offline. 

Does anyone know a solution to this? I'd think "delete oldest snapshot" would solve this issue, but it doesn't.

Thanks