check your Safari Autofill prefs, and see what you've got. you can edit individual sites to some extent there. then check Keychain, which is in the Utilities folder in Applications. further editing can be done there. you might need to delete the prefs for that site, and then log-in again to get the "save" dropdown.
or, as I think about it, I believe I have a banking site that makes me log in each time. a security thing I would expect.

formerly known as etaoin...