Welcome to the forum, @paulinho!
Vertical justification (spreading out the systems over the page) is controlled by the settings in Layout Options > Vertical Spacing > Vertical Justification. Almost certainly, your pages are less full than the first threshold in these settings. Lower it to something like 50% (or maybe even a little lower), and those pages should justify themselves.
Page two only has 1 bar because the first bar on p.3 has a frame break, which tells Dorico to start a new frame (page). Select that pink signpost and delete it, and those bars should flow back to p.2.
If you really want 4 bars per page, you might be better using the settings in Layout Options > Staves and Systems > Casting Off, instead of manually adding breaks.
Here’s a longer discussion of how the vertical justification settings work.