http://ephinea.pioneer2.net/faq.php said:Q. Where are all the "One Person" quests?
A. To open up more Single Player or "One Person" quests, you actually have to complete the Government Quests Momoka and Elly give you.
For example, if you don't do any of them in Episode I, you'll never see quests like "The Value of Money", "The Lost Bride", "Secret Delivery" and so on.
The more you progress with doing both Government quests and the Single Player quests, the more quests will become available for you to play.
You can only do Single Player quests one time per difficulty, at first, but once you do ALL of them in a particular difficulty, they will all unlock for you to play in that difficulty repeatedly.
Progress in quests is saved on a per difficulty basis. Unlocking or beating quests in Normal does not affect Hard mode and so on.