Sometimes it's the acoustics of a room, too; or bodies soaking up the sound (if there are enough of them to fill the room). I've finally learned not to be shy about getting a definite starting note or chord from the instrument, sometimes hitting the chord and swinging the guitar up close to my ear to hear it. It may look funny, but I'd rather hit the right note than worry about that. (I've seen Allen Damron do this in new surroundings or where the acoustics are "odd," but he's so smooth with it that it looks like a gesture of enthusiasm. Somehow I doubt that I'm that convincing...)

Sarah