Yes, you probably didnt work the area enough, put pressure in the area, lightly tap, etc, you know what to do, just pay more attention to the ass if you want to penetrate it with anything, its not like the pussy where you can pretty much dive right in, you've got to work it first to get her comfortable, also some girls don't like playing back there, you can normally save embarassement before penetrating by petting the area, in which she will either tell you or her body will respond in a way that you should feel like she doesn't like it back there, in which case you respect that and move on.