TAC would probably be the best route for resolving as they would need to look into the IMC logs during boot to see why it might be hanging.
There are quite a bit of variables:
-Type of server
-Standalone vs Intersight Managed Mode
-Firmware version
-Deployment method (Using the automated Intersight OS install feature or directly launching KVM and mounting the OS install ISO)
-Location, link quality, and mount method (nfs/cifs/https) of where ISO is
-Scope of issue, 1 vs many blades etc..
Without TAC and from a general standpoint I would look into latest firmware/SCU, different location to host the ISO (close as possible to the IMC), and continuous ping from where the ISO is being hosted to the IMC IP address to see if its dropping anything
