Are there frames in the timeline where you are trying to click after the playhead (not empty timeline slots)? You can't move the playhead beyond the end of the populated timeline.
The frames beyond playhead are not empty. It just can't move even I press the frame after the playhead.
What is the frame number where this happens?
It happen in the middle of whole timeframe. I encounter twice. One with a file of 1000 frames. It happen in frame no 850. The other with a file of 1200 frames, it happen in frame no 980.It happen when I copy something into the frames, it happen seems I copy too fast that make the file crash. I deleted
the layer and recopy again, it goes to normal.
Are you using CS4? An AS3 file?
It could be performance/memory related. If this is the case, try switching to AS2 and see if the same thing happens.
**Most importantly make sure you're updated to 10.0.2 **
If these don't resolve it, we would need to try reproducing your crashes internally with your FLA to figure out and fix the cause.