In older homes, or those that may have settlement issues. the joint where walls meet ceilings can often be anything but straight. I've found that if I try to force crown molding into a wavy environment, I may secure the molding but, more than likely, it will appear twisted or bent. Here's a simple technique that can help get you around that problem.
Cut small triangles from scrap wood, apply some construction adesive to the back and set them in place where you can locate a wall stud. Most homes have a two by four running along the top of the wall called a top plate. You can also nail into that. I strongly recommend using a brad nailer for this job -- it's so much easier and faster than hammer and nail and avoids any wall damage from pounding or missed hammer blows.