Nexus 5 lollipop update notification not showing up
Google releases their OTA updates in waves. If you have dismissed the notification (on purpose or by accident) you may not be eligible again until the next wave cycle. Many users have reported significant wait times.

If you want to update manually you can do so by flashing a factory image, although be warned, this will wipe your data.

You can also sideload the update files, which will keep the data intact.