The colored rings around your Tiles in your app change, depending on their connection status.
Here’s a handy guide you can reference if you forget what each kind of ring means!
Solid Green Circle: Your Tile is connected and you can ring it.
Dotted Green Circle (Spinning): Your Tile is nearby, but it hasn’t connected yet. Walk around until it becomes a solid green circle. You can use the proximity sensor if you don’t know in which direction to head!
Dotted Green Circle: You’re close enough to connect to your Tile, but you’re already connected to too many Tiles! Tap on the Tile to connect to it. Remember: doing this will tell your device to disconnect from another Tile.
 
Solid Grey Circle: You’re out of range of your Tile and can’t connect.
Solid Grey Circle (Spinning): You pressed the “Notify when Found” button to signify that your Tile is lost and you would like to be notified when it has an updated location.