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)