Selecting Drive

Hi, I have a 2001 Mercedes Benz c2400 auto, sometimes if I select drive it, ll accellerate but sometimes its like stuck in first gear and when I ease off the accellerator it still wont change up, now this doesnt happen all the time, when it happens I have to stop select neautral then select drive again then it appears to be ok, ive driven autos for years, but this is all new to me, thank you