Error 82 when adding drives to disk group in MD3820i

I have a MD3820i with a single Raid6 Disk group setup on it.

I recently added a MD1220 and I've been adding the new physical disks to the existing disk group until I received the following error:

Return code: Error 82 - The operation cannot complete because either 

(1) the number of physical disks selected is not valid for the RAID level of the disk group or
(2) the number of physical disks in the disk group is not valid for the proposed RAID level.

Operation when error occurred: Add Disk Group Capacity

I was only able to add 7 of the 24 drives, there are 17 drives that I can't add because I get the above error 82 when I try.

Anybody know why this is happening and how I can add the remaining drives?

Thanks.

James