Dorico 4.3: When I look at a page, it seems that there is room in the music frame for another system in the frame on that page (the right hand page of the screenshot), yet Dorico has cast off the next system into a new page. The page cast off occurred when I was in input mode, and entered a note on a staff that required many ledger lines. This was in the wrong octave, so I adjusted the note to where I wanted (which put it back in the staff), yet the subsequent system did not return to the page. See screenshot #1 (pages 7 & 8 of the score).
For the reason why this happens, see this traditional explanation by Daniel (here in response to one too many systems being in a frame, but the converse – one too few, like here – can happen for the same reasons
For an in-depth tour of the options available to you to work with Dorico’s spacing capabilities, see these two videos (helpfully thoroughly time-stamped)
(from around 39 mins in to this first one, John does a really good breakdown of how vertical spacing is totted up and compared against the available space inside the frame)