I bought a split V tunic top at Talbot's, and it arrived yesterday. I wore it out to dinner last night.

Sometimes, as with this top, the split neckline doesn't stay flat. It falls open at the top and even folds over. It looks kind of sloppy to me, and doesn't show off the neckline as well.
What causes this? Is it the fabric, the way the neckline is cut, my body??
Is there a solution for this particular top?