Why does the first frame change when I edit the second frame?

I am new to the Photoshop animation feature (in C.S. 5.5)., and I am having a problem.  When I edit the layers in the second frame, the same changes occur in the first frame.  If I add additional frames, they all change whenever I edit one of them.  As a result, all of the frames are the same, which makes for a rather uneventful animation. I have tried unchecking the "propagate frame 1" box, but that has no effect.  What am I doing wrong?  How can I limit the changes to the selected frame only?  Thank you to anyone who can help with this.