Why doesn't the screen on my evo 4 light up to texts anymore?

i got my sprint evo 4G in december, and it works great. except one thing: the screen used to light up when i received text messages, and for the past few months it hasn't. ive been looking online for answers.. but everybody says its not supposed to? well im not crazy. i know for a fact it used to light up, so why doesn't it anymore? help!