Students are not able to see the instructor's comments until after they've done their own self-review. This is the standard workflow, because we want students to use the skills they have learned from the Writing Workshop to evaluate their own work before they see what the instructor (or peers, if you're using the peer review feature) has to say.
Once all reviews are complete, they will be able to like/dismiss all the comments and make edits to the text itself. Then they can submit that draft for the next stage of comments/editing.
The best way to find out what's going on with your students is to ask them if they see any text next to the "submit" button. This usually reveals exactly why they cannot move forward. They can also look for the orange "Reviews Pending" notification at the top left, and read the "Instructions" (via the button, top mid-right) to find out more about what to do at the particular stage they are on.
Here are two screenshots of what the students could be seeing at different stages.
1) Reviews (instructor and/or peer) are pending (note the orange banner at the top). The student's essay is on the left side of the screen, but it is locked and cannot be edited. Self-review is complete and like/dismiss is available. However, the submit button is not activated (it is grayed out) and the message next to the button explains why.
2) Reviews are not pending. Student can edit their work (the essay is in an editable "box") and like/dismiss comments. All comments must be addressed before the submit button will activate (turn dark green). The message next to the submit button explains what to do.