Roster imports will generally only with master schedule imports. If you are unable to successfully import a roster file, most likely you built your master schedule in Illuminate directly - you did not import a master schedule.
So why is this an issue? Well, in Illuminate we create an internal section ID. This is the ID that you generally see on the screen when you are manipulating schedule data. However, when data is imported we make use of a "local section ID" column. This data is only ever populated when a master schedule is imported. Here is how it works ...
You provide a master schedule file with two rows: section A123 and section A897. When Illuminate imports that data into the master schedule data structure inside of Illuminate the Illuminate section ID is incremented based on the next value and the section ID that is provided in the data gets imported into the local section ID. So you data goes in as: section ID 1, local section ID A123 and section ID 2, local section ID A897. Then you provide us with a roster file for all the students in A123 and A897. The import process looks into the master schedule data storage, finds the appropriate local section ID, and then creates data accordingly in the student rosters.
So what's the solution? If you're completely done importing your "old" data and you have fully switched over to ISI, it's perfectly fine to "blow out" all of your old local section IDs and realign them so that they match the internal Illuminate section ID. This is not something that you can do, but you can contact Illuminate support and ask them to take care of this for you. Once this is done then you can import roster data using the section ID that you are now used to seeing.