Make sure your Bluetooth and location services are turned on, and you have an internet connection (data or wifi).
Hang tight, it can take a minute or two for your Tiles to connect.
Tap on the Tile icon for your lost item and walk around. When the circle around the icon is solid green, tap Find and your Tile will begin to ring.
If these tactics don't work, try restarting your phone and going through this process again. If you're still stuck, contact us.