Plugin Update Notification Not Showing

@solid_snake Upgrade notifications are checked by WordPress system not by single plugins, and it’s not clear if you miss all plugins notifications, or only those of NextGEN.
Also, this issue can be caused by a plugin conflict, so you should try to deactivate all of your plugins – maybe as a test you could downgrade a recently updated plugin, so you can see if the notification appears when you have all of your plugins deactivated.