There's been some talk recently of the upgrade advisor not being available in the Store on various Windows 8.1 phones or if available it will state the upgrade is not available. This was my experience recently with an unlocked 640. At one point it was giving the upgrade but then it stopped giving it over the past week.

Note: some windows phones will not get the upgrade and this can be googled.

Anyway, I've noticed as of this morning the upgrade advisor is in the store and is now giving the upgrade. I can confirm it works with unlocked 640 and I've heard it works with lumia 930 (that it wasn't working with previously) so if you tried recently and your phone is upgradeable and UA did not offer the upgrade, you might want to try again now.