One thing that happens every release is that Apple rolls out the new os in stages. If you imagine Apple has one thousand update servers, it starts with a few and then rolls out the catalog of available updates to more and more servers.
Anecdotally, it’s rare for an update check to not see the update within 24 hours in the US but I certainly have seen some cases where a non-major update (say 10.2.1 to 10.2.2) didn’t fully propagate or be offered for almost 4 days after “release” that eventually was offered.
If you need an update, using iTunes to force it is one option of your over the air server isn’t ready to provide the update you expect. Before resorting to iTunes or just waiting, I would also reboot your device once or twice in case it’s just some corruption on the running os and also free up 4 GB of storage if you might be running low on space to hold the download and data that needs to migrate during the upgrade.