The Nexus devices(and some non-Nexus devices)have begun getting their updates to the latest version of Android. Android 5.0 Lollipop is finally upon us and those over the air updates are rolling out as you read this, and if you haven't already gotten the update yet there's a chance it could show up in your status bar by the time you're done reading. That's good news. What users definitely want to pay attention to though is that if you have modified the system on your Android device in any way whatsoever, like giving your device root access for example(which changes up some system files)the OTA update will fail.
As Android Police points out this seems to be happening on the Nexus 9 and Nexus 6 only, which are both devices that were on Lollipop out of the box. However they received new updates to a newer build of Lollipop that brought some bug fixes and this plays a role in the issues. We reported on the presence of those problems earlier this morning which you can check out here. Essentially it doesn't seem to be affecting those older Nexus devices who updated from Kit Kat, but that doesn't mean the same problem won't come up later on once a new Lollipop build goes out for devices like the Nexus 5 and 7 as well as the others.
Without getting too technical(and to be honest that's partially because I don't understand a lot of the jargon)part of the reason this is happening is because the new way that the OTA script patches files in Lollipop is one giant cluster. Prior to Android 5.0 Lollipop, the OTA script patched files individually which allowed for modders and tinkerers to have rooted devices and still receive the updates over the air. What does this mean for you if you have a device that is modified? You'll need to be on a stock image that hasn't been altered before getting a successful OTA. Although this may throw a kink into the way rooters like to do things, the process for having the OTA script patch files as a collective group is a much faster process, which is why Google made this change in Android 5.0.