Missing vertical scroll bar on my frameset page

Hello all

I have an index.html set in 3 horisontal sections where the middle suppose to be the content container. Ideally, i’d like to have quite wide middle frame so there wont be a vertical scroll for that particular frame.

so what i did, i extend the height of the middle frame and it obviously pushed the position of bottom frame in the hope i can still see it by scrolling down the browser.

But for some reason, the browser vertical scroll is not showing, and i can only see half of that bottom frame.

Anyone know why is this happening?
Thanks