You’ve not really given enough information to diagnose the problem. It might be that you’ve been tweaking stuff in Layout Options without understanding the consequences.
With factory defaults, I get this:
If I tweak the flow heading margin, as so:
I get this:
Note that what’s actually happened is that the Vertical Justification settings (on the Vertical Spacing section of Layout Options) have kicked in - that few mm I’ve added has tipped the total amount of stuff on the page over 60%, so Dorico’s spread it out across the page.