I think after awhile you may build up a tolerance to it? Because the same thing happened to me. Three summers ago I had a major depressive episode, and what got me out of it was an increase of my lamictal (up 100mg to 300).

I like to think of lamictal as my back bone med. Without it I don't know what I would do. To me it feels flat and chilled out, but not flat in a bad way. I still have very volatile emotions. I can't imagine any med turning me into a zombie! I still get pissed, happy, sad, etc. And it keeps the depression away.