Hello Kate,
When my wife(Flo) and I started diving Flo had a very difficult time with bouyancy control. In her defense, she is very athletic and small frame. She is like a cork in the water. We tried everything the instructors knew to do to #1. get her down in the water and #2. keep her down. They wound up just loading her with weights. As we got a few dives under our belts she got a little better but still was not in that comfort zone. We found a divemaster teaching a Peak Performance class and that did the trick. He mainly worked with her weight distribuition and breathing. The weight trick, before intergrated BC's, was to put half the weight on her tank. This allowed her legs and fins to come up more horizontal position. The breathing came with practice and now she has excellent bouyancy control. As Fishkiller stated, this also helped her gas consumption. I highly suggest a bouyancy control class with a very patient instructor. It is a very frustrating experience for the person having the control problem. Take it from a dive buddy(husband) who knows, the frustration doesn't stop at the end of class. Thank goodness those days are behind us. Good luck.
Burt