The early builds shipped by mistake with JWR66N with developer keys so there was a mandatory emergency fix that installed JWR66N with release keys.
Then after that you would get the JSS15J ota update.
In your case it sounds like they started shipping the units with JWR66N with release keys so that makes the first mandatory update unnecessary.
Also google just pulled JSS15J last night so you wouldn't get the second update either.
JSS15Q replaces JSS15J but it is on phased rollout right now and if your unit hasn't been chosen no amount of checking for updates will get you that update.
So that is my guess as to why you haven't got any updates.
Disclaimer: No touchscreens
were harmed nor injured during the creation of this post. GPS
units were all treated ethically and humanely.
Signed: People against misbehaving Nexus 7 2013s