It looks as if you created messa di voce hairpins rather than individual hairpins, and those are by their nature more limited in terms of the edits that are possible (you can essentially change only the middle dynamic).
If you want complete flexibility about how the dynamics will be played back, I’d suggest drawing in some dynamics manually using either the pencil or line tools. You can “overwrite” the printed dynamics in the score in this way: the dynamics in the score will remain where they are, but Dorico will play the custom dynamics you have drawn in their place in the Dynamics editor.