Adding disks to an aggregate

Morning,

I am trying to increase the size of my aggregate. On a FAS8020, running 9.2 OnTap

rg0 is currently 23 disks with 6 empty  slots.

I tried just adding disks into those empty slots and get this error: Cannot add capacity to the aggregate because of insufficient matching spare disks.

When trying to add disks and create a new raidgroup I get the following error: command failed: Addition of disks would fail for aggregate "aggr2" on node "netapp-02". Reason: 29 disks needed from Pool0, but
no matching disks are available in that pool.

There are 43 disks available on "netapp-01" and 38 available on "netapp-02"

I must be missing something. Max raidgroup size is 29, so not sure what I'm missing.

Any help would be greatly appreciated.

Thanks