Unable to edit after teacher comments

I’m using the free version through Canvas. After I make any type of comment (actual comment, text, etc) my students cannot edit their original text. They can add new text, but they cannot change what they originally wrote. Unsubmitting and refreshing does not work for all of the text - it only seems to work for new text that I have not commented on.
I need them to be able to edit their original text as we learn to edit and rewrite. What can I do? Would upgrading help this?