^ ^
I don't know if this is fact, but when if you do have it in your Pokedex, and your tracking it, try not using fly, or the bike, I'm not sure, but when i was trying to catch Latias, those were the procedure i used.
Since it's random, it's very odd, but all the Pokemon games I've played with random encounter legendaries, i have always found them near my house, like where you catch the weakest of Pokemon.
So the only tip i have for you, and this is sorta of the method i follow, is grab a few Pokemon you would like to train, then train them in some area where there is grass, and your bound to found Latias.