Hey 101--are you talking about a 360 spin on the snow, or a spin in the air off a jump? From your question, I'm assuming you mean on the snow--correct me if I'm wrong.
If you're getting stuck at 180, and can't get turned back around the rest of the way, it's likely that you've been leaning forward (uphill) once you've spun around backwards. It's a common mistake, leaning up the hill for security. The key is to stand up straight or even lean back a little once you're skiing backwards, to put some pressure on the tails of your skis and lighten the tips so that they can swing around again. It's simple, but it isn't always easy. Leaning back when you're traveling backward down the hill can feel a little insecure, to say the least. But keep trying. Once you get it, you've got it. Besides being fun, it will develop skills that will make you a better skier all around.
You might start by just sideslipping down the hill, experimenting with movements forward and back on your skis to get used to the sensation and the effects of pressuring your skis in different places. See if you can pivot your tips down the hill from a backwards traverse across the hill. Then work your way up the full 360 spin. Be sure to practice on smooth groomed runs, because it's easy to catch an edge and fall hard when playing with flat skis and sideslips on anything rough.
Have fun, and good luck!