1) Open Failover Cluster Manager
2) In the left pane, select Cluster Shared Volumes
3) In the main pane, expand the CSV's. Make a note of the free space for each CSV and who the current owner is.
Although you have assigned the CSV more SAN storage, Windows sees this as unallocated space. You now need to tell Windows extend the volume and use the unallocated space.
As it's highly likely your Hyper-V hosts will run on Server Core, you will need to extend the CSV using the diskpart command or on another server by opening Server Manager and remotely connecting to the Hyper-V host and opening Disk Management.
5) RDP onto the Hyper-V host which is the CSV owner (as determined in step 3)
6) Open command prompt and enter the following bolded commands
a list of all volumes on the Hyper-V host will appear. Make a note of volume number for the CSV you want to extend (i.e volume 5)
select volume 5
You should get a message to say Diskpart sucessfully extended the volume
7) Reopen Failover Cluster Manager > Cluster Storage Volumes, and you should see the CSV has recognised the change.