A session is missing from the app

In almost every case the session is still safely on the sensor and simply has not transferred yet. Work through these in order.

1. Was it actually recording?

The light is solid red during a recording. If it was off or blue the whole time you were on the water, the session never started. This is the most common cause by a wide margin.

2. Bring them close and open the app

Wake the sensor, put it next to your phone, and open the app. Leave it in the foreground for a minute. Transfers do not run reliably with the app backgrounded, because both iOS and Android throttle Bluetooth work for apps that are not on screen.

3. Check the battery

A sensor low enough to refuse to start cannot transfer either. Charge it for twenty minutes and try again.

4. Restart both sides

Force-close the app and reopen it. Shut the sensor down with a two-second button hold and switch it back on.