Here's a short checklist:
Is Bluetooth on? Are Location Services on?
On iOS, you also need to make sure that Location Services are set to "always" for the iOS app. Estimote iOS app no longer requires Location Services to detect beacons for configuration. Location Services are still required in the demos.
[iOS only] Have you changed the UUID from the default one ("B9407F30-F5F8-466E-AFF9-25556B57FE6D") to a custom one? If so, then on iOS your beacons will only show up if you're logged in. (On iOS, apps are required to know the UUID of beacons they want to detect. So our app requires you to be logged in, so that we can download your custom UUID from Estimote Cloud and scan for your beacons.) We now show all Estimote Beacons regardless of the UUID!
Does logging out and in again help? Does reinstalling the app help?
We're currently aware of and are working on fixing a few bugs related to in-app session and cache. As a workaround, re-logging and/or re-installing the app usually helps. Said issue had since been fixed.
Does replacing the battery help? Sometimes the beacon has simply run out of juice. To replace it, you can pull the beacon out of its enclosure through the opening in the back, behind the tape, and insert a new CR2477 battery.
Nothing helped? No worries: email us to firstname.lastname@example.org, and include your Estimote Account email and the color of the beacon you're having trouble with—we'll work with you to figure it out!