I think the ones who can’t do new iterations are using the web site only, no command line. (Only a guess integrated from various memories…)
When a student is using the web site, I believe there are two different user interfaces / ‘places on the site’, one where they are ‘working on exercise X’ and another where they are ‘looking at the mentoring session on exercise X’. Both of these have ‘edit or submit a new iteration’ type buttons; but – I think – these buttons disappear from the mentoring session when it is ‘closed’. And then they may not realize they can get to the same action elsewhere, in the ‘working on exercise X’ section.
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I recognize that it is problematic, in a practical sense, that I am doing this without having done much of anything from the student side. It makes me helpless regarding student UI issues. I will eventually remedy this by actually ‘doing’ some of a track … but not today…