All the "Kill this many" quests have a limited supply of the monster needed. And until you get them "all" you wont get the last area with an abundance of them. So one area that might be filled with bugs, will have 0 bugs, until you have cleared out the other areas.
Best way to know which this area is, is to go to the harvest tour, and see where the monsters hang out. If you notice that it is empty when you do the hunt, that means there are still certain monsters you need to clear out from the other areas before they appear.