Replying to Spin Help

Hey, my friends and I were skiing yesterday...That reminds me I need to upload the pictures...Anyways...and towards the end of the day I attempted some 180s (I'd been focusing on grabs most of the day), and I just could not get a complete spin, I had plenty of air but would only get about 1/2-3/4 of the way around and land on my edges...resulting in disaster. I was skiing on Atomic GS:9s...If anyone could give me a crash course on what I'm doing wrong or what I should be doing to pull of spins I would really appreciate it.