I'm working on the Origins of The Harpies quest, but it won't let me keep /digging for the other items.

Like many other quests in the world of Atlantis, this quest has a component of randomness. What this boils down to is that only the main storyline quest items are guaranteed to drop. You will find yourself unable to continue /digging once you have found all of the necessary items for the main quest. If you have not acquired all of the subquest items at that time, you will need to acquire them another way. Please keep in mind that while you cannot obtain them through digging, it is possible to receive them from another player or through the Market Explorer.