Expectations of npm server package availability

Trigger: Attempted to install angular 2 release candidate and it was unable to find the package.

Question:

When is a package available on the offiicial public npm registry but not on the proget npm server?

We will use the proget npm server as a proxy to the public npmjs registry. Therefore we configure our server to only point to the proget registry. What is the policy or rule on available packages in comparison to the public registry? The initial expectation is that if it's available on the public npmjs registry the proget server will also have it available. This seems to be the wrong assumption.

Is there a delay or policy against beta or release candidates or some other characteristic that we should always expect? or did we do something really wrong?

Is this specific to proget or we should expect this from any private npm server?

Product: ProGet
Version: 4.1.1

It's hard to say, there's also the possibility that it's a bug; please keep in mind the npm "protocol" is undocumented, and lot of odd quirks that vary from client version to version, so it's possible this particular package is constructed in a way that we didn't account for.

In this situation, the best way to see what the problem is, use a tool like Fiddler or Wireshark, and see which specific query ProGet is responding differently to.