I had some agents that were 'stuck' in the Update Assigned state and
could NOT get them to update. I tried refreshing and re-assigning the
update and re-registering and all. What finally kicked the update into
action was renaming the folder with the GUID under the system-update
logs folder. So, if anyone runs into this problem, try it. Rename the
folder and then do a refresh and it should kick the update off again.

The reason that these were stuck appears to be that the update
originally failed but for some reason did not report back to the ZCC
that this was the case and so I didn't have an option for clearing the
update and redeploying like you usually do.