When adding a new chassis the firmware shipped with the IOMs may be different than what you're currently running. The IOMs will automatically download the matching fw version from the FI's - so just be patient once enabling the server ports for the new chassis and blade to discover. Could take up to 20mins (to pull upgrade image and reboot if necessary).
Other than that, ensure all IOM - FI links are acknowledged. Once the chassis is discovered you can re-ack the new Chassis just to be sure.
Regards,
Robert