Chasing the Truth not showing up

I went to Lago Zancudo, there is a gas station where I fortunately found a tanker (what trucks use to carry gas). I parked the security van next to it and I shot the tanker from a distance. Four shots later it blew up and open the backdoor, dropping a brief case with $7,000!!!!

Go the the wiki and it explains everything.

Can be ordered on a special in-game app after completing Franklin's intro mission. Has to be picked up at the location give on the map. One of the most common locations for pick up is at Los Santos International Airport.

So you have to go to that location. The woman just says she'll have it ready for you on the pad. Not come to you.