Cant see own code too few lines

Hi all,
my current issue is silly but very real.

When working with AS3 in Flash CS4 the actual size of the window that I edit the code in does not show all my lines…
Doing page down never reaches the final lines… Is there a setting on how to change this somehow?

For example I have a file 150 lines of code - I can only see and edit the first 140. Very annoying.

PLs help