Which client are you using?
Just to be sure, last spell only works if you have dbl clicked on a spell icon. Spells that you have just cast using another macro will not be remembered by the last spell macro.
I'm guessing you have just set up the last spell macro? If so, you might have dbl clicked the enchant spell previously. Open your spellbook and dbl click on another spell, then hit your last spell macro and see if in casts the correct spell.
I use last spell for non-combat spells that I normally don't use, but if used, may need to be cast repeatedly - eg mark, rez, exorcism etc