Concatenated text measures causing 'locking conflict error message'

In Power BI Desktop, that is connecitng to a SQL Server database in import mode, I have some visuals that aren't loading due to an "Operation Cancelled due to locking conflicts" error message.

This is only happening on some visuals.

My data relates to teaching activities - I have what is like a "fact table" that contains only IDs; activity occurence ID, student ID, teacher ID. By occurence I mean that if an activity runs for 10 weeks - once a week, it has 10 occurences. If each occurence has 30 students attending, and 2 teachers delivering, there will be 60 rows in the fact table for that activity occurence.

I then have other tables (like dimension tables) for student details, teacher details, activity details whihc all have relationships to the fact table. 

I have some table visuals that work fine - these are ones that display the data at its most granular level - i.e. the same activity occurence will appear 60 times if it shows the activity name, student name and teacher name.

In other cases I need to group things together onto the same line in a table; I need to see, for an activity occurence, all teachers (concatenated) on the same row, and all students (concatenated) on the same row. For this, I use measures, as per below:

ALL Teachers = CONCATENATEX(DISTINCT('Teachers'[Name]),'Teachers'[Name],"; ")
ALL Students = CONCATENATEX(DISTINCT('Students'[Name]),'Students'[Name],UNICHAR(10))

When I try to put these measures onto a table visual, so the user can see an activity occurence, plus all tutors & students attending, in one row, that's when I get the "Operation Cancelled due to locking conflicts" error message.

The measures exist in my "fact table"

I'm not sure if I need to investigate this on the database side, or if there's something else I can do to get around this error. I need my data concatenated like the above measures to meet customer requirements.