I have two identical Nexus 7 (GSM) devices. Both running Android 4.4.4, KTU84P.
Now one of them shows that Lollipop 5.0.2 is available as OTA, but the other one says that no updates are available. How is that possible?
Rollouts are conducted in phases. Typically they start at 1% of devices for around 24 - 48 hours; we watch the return rates and resulting device checkins and error reports (if any), and make sure nothing looks wrong before sending it to more. Then typically it goes to 25%, 50%, 100% over the course of a week or two.
What the percentages mean is that when your device checks in, it has a 1% chance (for example) of being offered the OTA. If it doesn't (randomly) get an offer, it will never get an offer until the next batch.
Since the Lollipop update for the GSM Nexus 7 just began yesterday, one of your devices was likely chosen for this "wave" and the other was not. If you want to update manually, you could try to find a copy of the OTA and attempt to sideload it or use the proper factory images.