Definitely not manually adjusting!
Have a look at Layout Options > Vertical Spacing > Vertical Justification. These percentages are thresholds. If a page is less full than that, Dorico doesn’t stretch the staves/systems apart. If you set them to 100% you effectively turn it off. In your case I think you want it set much lower so it will work on these pages. See Ben’s guide to vertical justification for excellent illustrations of how these settings work.