Why won't certain batteries recharge?

I've had this charger about a year now.. and recently it won't charge some of my batteries (i have 12 total). Sometimes the red x flashes, sometimes the whole top display flashes, and still other times it will charge just fine. It isn't any specific batteries, the older ones actually do better. It is very frustrating when I put batteries in to charge, and later on when I need them they aren't charged at all. The are all energizer recharge