It's always possible, during a new iOS upgrade rollout, that the Apple servers are "busy." However, if after a few days, your iPhone is still not being offered the upgrade, then I would suggest that you try upgrading by first connecting your phone directly to a computer to help "force" the upgrade.
Also the support article you came here from should have provided you with information on the upgrade process. Did you review it already?
Also, one other thing to check is if you have a beta profile installed. This would need to be removed to receive the upgrade notification.