In the event that you fail a levequest:

After the leves reset talk to one of the npcs that issue them, once you do, the failed leve in your log will become 'uncompleted'.
You can now try to complete it again, however it wont count towards the ''8 leves per reset''.
Unlike leves you complete right away, you can complete a failed and resetted leve and imediately exchange it for a new leve.