How to start the Merryweather Heist Setup?

I am up to mission 30 in the GTA V story mode. The next mission I'm meant to do is mission 31 which is the mission where you have to steal the submersible. I for some reason can not get past this point. I've done shooting practice with Franklin and have finished flight school with Michael. I've done every single mission for every single character including the side missions. I've seen videos for completing the Heist setup and apparently there is meant to be a HS on the map for Trevor but for me there doesn't seem to be anything except side mission. Either I'm stupid and blind or the game is glitched. Or both. Trevor's GTA V Map


Solution 1:

The last part of the Merryweather Heist will open up for play after you complete several other missions as various characters, such as The Long Stretch, Hood Safari, and the third FIB mission. In order to complete the third FIB mission, By the Book, you need to receive a phone call while playing as Michael or Trevor (not Franklin). Please switch to one of those characters and make sure that your in-game phone is not off, so you don’t miss mission-triggering notifications.

Source