There are a few items that can cause upgrades. Most of the time if the box is in production and being used by clients the box will not have enough resources to complete the upgrade.
Typically I recommend disabling reporting and removing customers from the box to upgrade. This will free up the resources needed to upgrade.
What version are you upgrading from and too?
Christian Rahl
Customer Support Engineer
Cisco IronPort - Web Security Appliances
Cisco Technical Assistance Center RTP
United States Ironport: 1-877-641-IRON (4766)