Sonatype Nexus incompletely proxies external repository

I'm currently attempting to set up a proxy repository (as described in the Nexus guide) for the Bukkit repository, in order to build some dependent projects locally. I'm using the URL:

http://repo.bukkit.org/content/repositories/bukkit_repos/

It seems to be the right URL, but no matter my configuration, I can't seem to get Nexus to actually proxy any artifacts through the repository. I've tried:

• Using different IDs in the local Nexus version, both bukkit (my desired ID) and bukkit_repos (to match the remote repo)
• Changing the policy of the repo from Release to Snapshot, and even recreating the repo with the different policies
• Adding the URL above as a mirror
• Expiring the cache, rebuilding the index, and updating the index

No matter what I try, I can't get any artifacts to appear in the proxy. What am I missing?