I'm a relative novice on the 737NG, however when are you chopping the thrust and are your eyes moving towards the end of the runway in order to judge the flare? Another reason for a firmer landing is becoming slightly unstable towards the end of the approach i.e either lateraly or verticaly. Just trying to use examples described by various LTCs at my airline. Probably nothing new to someone with your experience on type. Wouldn't worry too much, landings will always be a bit of a 'black art' and unfortunately cannot always be perfect. Anyone who says otherwise is not only kidding you but also themselves!