I encountered the same problem yesterday - and found the solution this morning.
The Moodle workshop module offers two possibilities to switch from submission to assessment phase:
- you switch manually
- you specify the availability of submission and assessment phase in the workshop settings and tick "Switch to next phase after submission deadline"
If you chose the second option as I did it is still possible to switch phases manually. I did this to shortly test whether in the assessment phase I could see the assessment form as a student. I could not. The reason is that the settings from option 2 override the manual switch (option 1). So when I was testing, the assessment phase was not yet available for students - and therefore I as a student could not see the assessment form. As soon as the assessment phase was available for students, I could also see the assessment form.