Recently, we have been covering the rollout of Lumia Denim across the world, and some people have been commenting that no matter what they do, they cannot get the update on their Lumia phones, even if it is available on their device, carrier and country combination.
The explanation is simple: the Preview for Developers app enabled is getting in the way of this update. Some users have reported that you need to disable the app by un-ticking "Enable Preview for Developers" > "done" and then rebooting the device, but others like myself have had success by simply disabling Preview for Developers and then checking for updates without rebooting.
The above method is required for Denim.
Another strange bug, is that when you are successful in updating to Denim, and you re-enable "Preview for Developers", going to Settings > Extras & Info shows that Lumia Cyan is installed! Disabling Preview for Developers again and then going back to Extras & Info miraculously shows that Lumia Denim is in-fact installed. An odd bug indeed, but one that has been widely reported by users.
Anyway, we hope we have helped in some way to clarify the method for updating if you have Preview for Developers installed. Feel free to comment on your own experiences, and especially if you have the Cyan/Denim Extras & Info bug.