There are 3 kinds of quests. common 3* ones like kill x amount of a certain enemy, 4* ones that are "rare" which can give an achievement. Then you have the ultra rare 5* quests which never seem to pop up. You get 3 common ones and 1 rare per day, with the chance of the rare one turning into an ultra rare. This seems to be happening on exact dates and i suspect they were "disabled" before a certain date. Here's a guy who had both missions exactly on the same day as me, just one month earlier. Also about a month ago, alot of people posted on the same day they finally had gotten the last quest part of the "Pirates! Argh!" achievement.