When media is deleted off the NX2/ZX storage and the free space is not growing the issue more than likely lies with snapshots. The data will not be freed until the older snaps are deleted that are holding that space. The best practice to delete these older snaps is by adjusting the Snapshot retention plan down, it will delete the older snaps once it takes the new snap and once you are happy with the free space change the retention plan back.
To adjust the snapshot retention policy navigate to "Backup & Recovery", choose "Options" for the policy you wish to edit and choose "Edit Backup Task". Here you can adjust the retention rules, change how long the snaps are kept for. An example would be to set the policy to take a snapshot every 1 hour or less and keep it for 1 to 3 days depending on how quickly you want the free space, please keep in mind if you set it lower than a day you will be getting rid of all good snaps very quickly so please make sure you pay attention and set it back once you have the space you are looking for cleared.
