Meagan, it looks like the molding is not installed the right way... The crown needs to be reinstalled and that should solve the issue unless the ceiling is way out of whack but I don't think that's the case here (hard to tell from the picture).

If crown molding doesn't come in contact with the ceiling and the wall at the right angle, you will end up with an elongated gap like this. Here is an example to show you what I mean and how the Crown should come in contact with the wall and ceiling.

Thank you. Do you think I can just re-install the pieces that are out of whack and leave up the ones that seem to be fine?

Yes, run a utility knife along the top and the bottom of the molding where in contact with ceiling or the wall so you don't pull the paint with it and along the joint where the molding connects and gently separate the molding not to break it, pull any nails attached to the molding from the backside of it so you don't damage the surface and re-install the molding.

That being said, depending on the amount of molding you be taking out, in most cases you better off getting a new molding one size bigger, install it the right way and have a nice clean look.

Good luck