VirtualBox doesn't see the latest update

I think the problem is since v4.3.14 VirtualBox enhanced the internal security that caused problems with many anti-virus and other software (only under Windows host), eventually make all guests unable to start (I got the same problem too). So the developer disabled the update notification after that, currently it is not sure when it will be resume.

As mentioned by AlphaCluster, you can download and install the latest version manually. However be aware you might get the same problem.

Here is the latest discussion on this issue, look like it still have similar issue on v4.3.20.

If it was a recently released update, it may take a while for it to reach your installed version. I would try again in a while.

Since 4.3.20 came out yesterday that is likely why it's not visible yet. It's not uncommon for it to take a day or to before checking for updates picks up updates. I would recommend just download the latest version from https://www.virtualbox.org/ and just use that installer if you want the latest now.