It really is a case of using your legs to swing as far out as you can and then quickly round for maximum momentum. The further out you can get your legs at the beginning the more momentum you'll create when you bring them back into the bend.
Also try to notice how hard you're gripping, both with your hands and your underarm.
A looser grip will seem more scary but will allow you to go round more easily. I found that this was the reason why I couldn't do the Pretzel spin (for the same reason as you) and once I became conscious of how hard I was gripping it was easy to try to work on making it looser and thereby allowing me to spin.

Start a little further away from the pole when you do this move. Also remember to swing your outside leg around (keep it low) a lot of people tend to swing their leg up to the ceiling and it wont work if you do this. Swing your outside leg around like any other spin you do. So the key for corkscrew is, start further away from the pole, reach up and pull yourself up and keep your outside leg slow as you swing it around. Hope this helps xx