Why is this like this with just 2 systems on page 1?
I can’t see any overrides anywhere and I always delete all engraving changes. (Setting signposts visible doesn’t show any frame breaks)
In any event to avoid unexpected overlaps everywhere I’ve set casting to force 3 systems per frame
I tried engrave format |reset layout but nothing changed.
StringQuartet1.Move2.dorico (1.7 MB)
Try removing the Scale number… under Library→Layout Options→Staves and Systema→Casting Off, or Fixed number of systems…
Jesper
This is yours, simply with make into frame of the first three systems…
Also in D5, it’s OK:
StringQuartet1.Move5.dorico (1.9 MB)
Obviously Jesper’s solution is immediate! Really great to spot this in the manual!
If you activated Scale number of systems by frame height, the number of systems contained in each frame is adjusted according to the size of the music frame. For example, pages with smaller frames, such as the first page, contain fewer systems than your casting off setting.
thank you, as always, for some great answers. The root culpit was " Scale Number of systems by frame height." I guess the algorithm decided 3 was the number of frames in a full system and that first page, being ever so slight short of full system, only got 2. Integer arithmetic works like that and rounds down.
Isn’t it rather you who decided this in your Layout Options->Staves and Systems->Fixed number of systems per frame = 3?
you are correct. 
I decided 3 was the right number, because 4 overlaps systems
But what I didn’t realise 
was that the first page would get less than 3 simply because first page frame height is slightly smaller than the other pages.
You should give it a try completely disabling that casting off option. Let Dorico do ist calculations and have a look at the outcome.
A string quartet is too complex a music to get squeezed into a fixed pattern of systems.
Hi @aceastwood, beyond all useful suggestions above, here a version of your layout that doesn’t uses a fixed casting off and no manual adjustments (only a couple erase background for the dynamics, and a reset for the Stems directions. I also adjusted the indent for the System Dividers). Just a global Note Spacing setting (4 1/2), and a local Note Spacing Change (4) at bar 38. The music uses now all 4 pages:
For String Quartets (on A4) I find a space size of 6mm (size 5) reliably gives three systems per page. No other messing around required.
