Neighbors who have been sleeping for a long time will disappear from your game. This is due to Facebook's app policy. To put it briefly, games, such as K&B, if not used for more than 90 days, may become 'expired', and their users are no longer shown on neighbor lists in games. Read more.

If you know that your neighbors are active but they're still missing from your game, or your neighbors don't see you, check the permissions for K&B on Facebook:
1. Open the Apps and Websites manager.
2. Make sure K&B is active.
3. Click 'View and Edit' next to the game logo to check and renew permissions. The Friends List permission must be enabled to have neighbors and be shown as one.
4. Ask your neighbors to set up their settings, too.