I was having the same problem...
Exporting only showed the top 2/3 of my picture when I save it out as 300DPI
Want to know how I solved it?
I made a Photoshop file 6.627 and 10.25 at 300 dpi. The pixel size is 1988 X 3075.
Then I made my FLA file 1988 X 3075 pixels. When you save it out at 72 DPI... the whole thing shows up and fits perfectly in the PSD file.
So... there you go!