When you hit the Duel button from the main screen, you open yourself up to a few surprises: you don't know who the opponent will be, which of your unlocked fisheries the Duel will take place at, or the species of fish you will face! This matchmaking is typically determined by the number of Trophies anglers have gained so far, however, they also both need to have access to the same fishery for the system to match them.
As a result, if you recently unlocked a new fishery, you may still be building up your lure collection for that spot, so a randomly selected Duel there becomes tricky. Combat this by opening new lure packs, purchasing special offers, and participating in fishery Events.
Speaking of Events, they often require competing in a few Duels, don't they?
Remember: if an Event needs you to complete a Duel-related task, you may always hit the Duel button next to it in the Event task list. This will automatically select the fishery connected with that task for the Duel and can help immensely, especially when time is of the essence!