OTA updates for the Nexus family has started to roll out, but as the release is coming gradually, it might take a while to actually reach your device. You can tray to check manually for the update if you have not received the notification yet, but probably, it’s not working either.
No need to worry, though. Google has finally published most of the factory images for the Nexus family. Currently, there are images available for the Nexus 5, Nexus 9, Nexus 7 (2013) WiFi, Nexus 10 and Nexus 7 (2012) WiFi.
So, it means that if you have one of those devices, and you have not received any notification regarding the OTA update, you don’t need to wait for the notification any longer. You can just download the image and flash it manually on your device.
If you already know how it works, just go the the Google developer’s page and follow the usual steps. If this is the first time you attempt to do something like this, don’t panic. The detailed instructions to flash the images on the Nexus devices are posted on the same page. Read them carefully and follow those steps and you will be enjoying the new and fresh Android 5.0 Lollipop on your Nexus.
Finally, if you are interested in the binary files, they are already posted there for the same models.
There is still no information regarding the factory images nor the binary files for the Nexus 4 and Nexus 6 smartphones, but they will probably be available soon.