Vertical page breaks won't drag in Page Break Preview
 

I'm a massively experienced Excel user and I can normally fix pretty much
anything, but this has got me stumped.

I set my sheet to print on x pages by y pages as a shortcut to getting it
close to how I want it.
Then I set it to whatever % size Excel has estimated based on the number of
pages.
Then I go in to Page Break Preview and start moving my page breaks around to
exactly where I want them.

The horizontal page breaks drag just fine.
The vertical page breaks won't budge.

Huh?
Anyone know a get-round for this?!

Thanks!!