I can see Alexander in the 1881, 1871, 1851 and 1841 census, but can''t find him in 1861 at the moment. His birth date from this varies between 1824 and 1821.
It is possible that the entry has just not been made or that it has been lost from the register. I don't know the condition of it, but it is possible that the page is illegible. Or even that the minister forgot to do it. I see that Currie seems to have a register of neglected entries, which does impky that some were missed at the correct time. Maybe Alexander was missed even from the RNE!
Nell