I've imported a roster, but when players that are on that roster attempt to join via keyword whisper they're given the response that they're not on the nominated roster.

Thanks for reporting this. Does this happen with multiple people? Did you check that the spelling is exactly correct in your roster list? You can try adding the name of the player manually with the add button and typing it in, can you check whether the roster stays the same after adding (which it should because it detects that this player is already on the list)?

And I assume this is with the roster invite only setting activated?