Think about pulling the opposite shoulder from the way you are twisting in the direction you are twisting, also remember to turn your head in the direction you are twisting and then spot for the ground. - this is the technique i use (with my arms at my chest).
Another technique that works good and might be just what you need to get all the way around is after you set drop the arm that is the direction you are twisting to initiate the twist and then drop the second arm and sort of pull them towards the side you are twisting. I think they use this technique in trampoline a lot.
There's two different things you can try, but you will have to check with your coach and see if they have a technique they would prefer you to use.
Trying to set higher may also give you more time to twist when you are on the tumble track vs. the pit
Good luck, keep working at it, you will get it! When i was learning i had the same problem, i could never twist past the 1/2 but even though it seemed like i would never get it, i did after lots of practice so don't give up. Plus, you are almost there if you have it into the pit.