Really great level design, + and - mechanic was underused, but when it was used it was really cool. Would love to see it built out even more!
I see! that's because the last level on each page is a bonus with certain conditions to be met to unlock (hinted by the lack of star requirements but only ???) also there is a blinking arrow on the right of the screen as soon as you have enough stars to have unlocked another level in the next page, I hoped this fixed the misunderstandings but it seems that's not the case :(