If its actually a lack of heat I'm not sure, but does it sound like it is blowing strong and you just don't feel the heat coming out at the floor? If so it could be the diverter flap thing. Not sure what it is really called, but it changes the direction of the air, and apparently the existing one is not strong enough. My '08 does it. Seems to continue to mostly blow out the defrost (or just feels like it is blowing with no escape) even though it is set at the floor. Someone had a breakdown of the repair, and it seemed pretty involved. I think its a fairly common problem.
I'm sure someone will post that has a lot more knowledge of it than me, my details of it is pretty vague.