CX4-480 - Unable to expand disks on RAID Group

When clicking expand on a Raid Group 1/0 the expand button is not enabled.  I would like to add disks to it.  The existing RAID Group has 16 disks. We only have 11 disks free, I would like to add 8 disks to this existing Raid Group.  The disks that are free are the same drive type and same space. Any idea why I am unable to expand and add those disks? If I try to create a new RAID group it allows me to select those 8 disks. Thanks!