I agree that it's quite normal to lose a maneuver that you were previously good at once you start concentrating on learning something new. One factor is simply the amount of time you spend practicing a particular maneuver: once you started concentrating on a one-foot spin, you likely didn't spend that much time practicing a two-foot spin. Another factor is the balance and overall body position for a one-foot spin is quite different from that for a two-foot spin. For a one-foot spin, you've probably spent a lot of effort centering your weight over the spinning foot. Your body has gotten used to it. Then, when you try to center your weight more evenly between two feet, your body fights it. If you want to maintain both a two-foot and a one-foot spin, you'll need to keep practicing both regularly.