Usually when I don't know what to do I click on every area in both day and night cycles, and that usually works. I can't remember what the exact trigger was for this event. I was stuck at 90, clicked on options I didn't think would work, Korra disappeared, Wife and Lin told me to look in Arena, was automatically sent there, a fake Amon scene happened, She said she would listen more, I talked to her at one point and she said something like "I'm not into being more open-minded with my training lets do it some other way, like psychically", night event with Amon mask on, 90 cap was gone.
Some things you could try to trigger it that can think of is: Train Korra in airbending, you need a scroll from Shady to do that and get to Level 5 in Spinners. Be mean and buy equipment to make her happy? Click on the star to do Probending. IDK what else could be the trigger.
When she went missing for me I think I encountered a bug, I clicked on the star and did a Probending match and thought "Ok, found her" but then she was still missing in areas she would be in, talked to wife and that furthered the event with Lin.
