How many ESX servers do you have? If you have more than one you need to ensure all hosts that have access to that LUN under vCenter have been rescanned. If you look at the datastore properties for that volume under each host you need to ensure they see the proper size of the LUN, otherwise vCenter will not allow you to grow the LUN. You can test this theory by finding a single host that sees the correct, increased size and logging into it directly with the vSphere Client and increasing the size of the datastore there. Once you have done this you can rescan the cluster and all hosts should reflect the new size.
Regards,
Corey