I’m having real bother with vertical spacing in the attached score. As far as I am aware, I have altered none of the defaults for staff spacing - the only changes I have made to default settings at all are changing the staff size, making the text size for lyrics larger, switching on Hide Empty Voices and forcing one system break (to coincide with the entry of the soprano solo).
Why, therefore, do I have overlapping systems on pages 3, 4 and 5? I would expect to be able to fit two systems per A4 page, but it’s a pain to have to toy with the settings with the music so obviously incorrectly laid out.
Any advice appreciated - I wonder if I’ve run into a bug or if this is something stupid that I have done. Many thanks!
[N.B. Edited to remove original score now solution has been posted]
One thing to try: adjust the bottom edge of your music frames on those pages up (making the frame smaller). Do this a space or two at a time until the system justification kicks in. Weird, but effective.
Go to Layout options and reduce the Space size from Rastral Size 6 to 7, and voilà … 
The reason you get staves overlapping is that Dorico has to estimate how much music it can fit on the page while it’s casting off. It doesn’t know at that point exactly where everything will end up, and it thinks it will be able to fit two systems comfortably onto pages 3, 4, and 5, but the fact that there are dynamics above all staves on all systems prevents this from being true. I agree with Frank’s advice that reducing the space size on the Page Setup page of Layout Options is probably the best remedy.
Many thanks, all, for your replies. You’re quite right that it spaces really well when at Rastral Size 7 - I’d hoped not to have to take it that small, but I should have tried it before posting on here!
Daniel, I’m sure this is on the ‘wish list’, but in future, it would be fantastic if Dorico could somehow recognise if there are staves overlapping once it has cast off and then bump the next system onto the next page to avoid this overlapping. I don’t wish to sound ungrateful, though - I’m absolutely loving using Dorico and it produces gorgeous results. Thank you!
I agree it would be good if Dorico could resolve this kind of situation completely automatically, and in the future perhaps we will be able to come up with a good solution for it, but there is a circular dependency between page layout and casting off, which means that any change to one can have a knock-on effect on the other, and that does make it more complicated. The current situation is a compromise between functionality and performance.