Adding new frames at the beginning




Struggling hard to with no luck right now.

I had created a couple of layers, som keyframes and som tweens and also an audio track (don't know if that makes any difference)
But now I wanted do push all my already created work a number of frames ahead - so far so good, that was not an issue
When I did that I got a number of empty frames - spanning from the beginning to the frame to where I pushed my existing frames...
I thought I'd then only have to insert new keyframes on the first and last frame in that newly insterted span...
But when I create the first fram as a keyframe and insert content, then same with the last... all intermediate frames remain empty - I'd expect that all frames would look the same after the first keyframe until next keyframe where a change would take place (had I done any Changes in that last keyframe of course)

What am I doing wrong here?