EDIT UPDATE.
I figured it out. There wasn't an obvious button to implement the submission phase, so I mistakenly thought I was already in that phase. It turns out that I wasn't, needed to push the little "ligthbulb" icon. That put the class into submission phase.
All good.
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So then something is wrong in my situation as I think I've reset everything back to the submission phase.
But Students are still unable to edit, or add to their submissions.
So, to be concrete, how would YOU reset back to the submission phase, if you had to?
Thanks David