Try a server reboot. Sometimes the package refresh can get hung and a reboot will resolve this. This is rare.
Check your package servers. Do they all show 'Ready' status for this package?
Check the package assignment to be sure it's being assigned to all sites. If 'Assign package to: Package servers by site' does not have this agent's site's package server selected, it will not download the package, even if the agent needs it. As a result, when the agent requests the package, the NS cannot provide a source location. Again, if you've checked and the package is on the package server and the status is 'Ready,' this is an unnecessary step.
Be sure NSE's are processing normally, since performance issues could cause the NS to fail to respond with a codebase. Sounds like other sites are fine, so this is unlikely.
Wait 24 hours. Often in cases like these, some scheduled tasks need to occur before the NS can provide the agent with a package download location.
Do any of these steps help? If not, just so I understand correctly: this is the only package that doesn't work, and this is the only site it's not working in?