It turns out that there are a few important factors we sometimes forget about over time. Once you've done hundreds of them, and you stop thinking about each little component that makes the turn complete, you form a tendency to pick up bad habits or forget important techniques.
One of the most important things to keep in mind is to keep your body straight and vertical, perpendicular to the level of the beam. When you lean forwards or backwards while turning, it'll feel like you're out of control and it throws off your balance. Pretend you're a marionette (those puppets controlled by strings). Each time you go into a pirouette, pretend someone is pulling straight up on string attached to your head.
Most of the time, gymnasts lounge before initiating the turn. There's nothing wrong with that, but sometimes gymnasts lounge too much. If the lounge is too wide, your body tends to lean forward a little, and it requires much more effort to pull up into the vertical alignment. Experiment with your lounges and find a comfortable width to work with.
Judges are very picky about the position of your free leg. Be sure you hold your leg in a position that makes it clear about how it's intended to be. If your toes are on your ankle, or if they're on your knee, make the distinction clear. Or if your leg is turned out or turned in, make that clear as well. If the judges can't tell what position you're in, they will most likely take a deduction.
This is definitely a skill that requires repeated practice until you find the techniques that work for you. Keep trying, and good luck!