This is [status-bydesign-but-will-probably-be-improving].
Basically, when we haven't seen you for a while we drop your inbox out of memory*. The instant we see you on any site we start rebuilding it, but that can take some time (generally, less than 5 minutes). If you come to the site (after a prolonged absence) and immediately check your inbox you run a non-trivial risk of catching it "empty" ; naturally, improving this is a mid-term goal.
To give you an idea of the scale, the global inbox has been live for less than a month and it already has 1/2 million persisted entries. Holding this (naively) constantly in memory would eat about 1/8th of our network-wide cache, and it's growing rather rapidly.
* Or if we experience any technical issues that force us to restart our servers.
A little before our datacenter migration the way inboxes are loaded was changed, it should have made loading much more reliable in the face of load. Now that everything's settled down, I'm pretty confident this problem has been resolved.