Why won't my action replay codes work when I try to use L+R codes?

I got a new action replay dsi for my nintendo ds lite today... all the codes are working... except for the L+R codes. I go in game (pokemon diamond) and try to press l and r a t the same time for a second. Nothing happens. 5 seconds. Nope. a whole minute. Nata. alternatively? STILL WON'T WORK. Help me, please. What am I not doing right?