Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
Save while you are offline and the BuzzNav doesn't show the red mailbox icon. Go online and get some fresh StreetPass data from passersby. Check if you got the Mirage Spots you're looking for. If not, soft reset and go online again to get fresh data. Do this until you find the Mirage Spot you're looking for.