Hi Mary,
Thanks for responding. I've resolved the issue (not sure if it's a bug or whether it's designed this way?).
Essentially, if I have the restrictions as nested in a "restriction set" like the one below, then the only learners available for override are those who have the matching user profile field:
However, if I break the restriction set and separate the user profile field restriction as below, then all the students are available for override.
I'll leave this here in case it is useful for others too.
Cheers
Andrew