Ok, now that we know it works properly in a standard project, we need to try and find out why this is happening in your other project. Could you check and make sure all your sprite collections are built properly. This could possibly be caused by one of your sprite collections getting broken for some reason or other.
The next thing to test is, in the project where its broken, simply load the first demo, and check if it works there, and then the multi atlas demo and test it there.
Finally, delete the -tk2d file in the root of the project to make sure it reloads it.
Let me know how it goes...