Sometimes that is the result of the prey item not quite being thawed all the way through, or it was slightly too large, or something startled the snake before it could really start eating. I've seen it happen once or twice where the snake couldn't figure out where the head was, and rather than keep trying after a few minutes just gave up completely on eating.

If it doesn't feel secure, it may also coil but not eat. Do you have hides in the cage for it? Sometimes, adding a hide or changing the existing hides can help.

-Jen