I am currently working on a full orchestral work, and when I add rehearsal markings (bar numbers) to the score it shows up fine, but when I go to edit any parts, only the box appears, not the actual bar numbers. I’ve tried to change the staff size but that does not change anything. The same problem was occurring on my wind quintet piece, but I was able to fix it by keeping the staff size as a certain length. Does anyone have any ideas? Help!
Do you have any local time signatures in these parts?
And welcome to the forum! 
Welcome to the forum @jawaters1006 – just in case you weren’t aware, there’s a newer version of Dorico 4 available if you want to update.
It could be a font issue. Try changing the Font Style for Rehearsal Marks, as a test.
Otherwise, if it’s happening in one layout, but not another, then it’s likely a Layout Options problem.