The two usual causes are a dead battery, or not enough valve thread to trigger the sensor pin.

A dead battery just needs replacing. If the thread is the problem, try fitting the sensor without the security nut — the extra thread is often enough.

To force a sensor into discoverable mode, either:

• Slowly unscrew the sensor from the valve. The pressure difference should make it broadcast.
• Or remove the sensor battery, wait a minute, refit it, and start the scan again.

The scan lives in the Devices section: Add New Device → DMD TPMS, or the + on the TPMS card if you already have one paired. The dialog is called Add Sensor and it keeps scanning while it is open — keep the sensor near the head unit. Sensors you have already paired are filtered out of the list, so a sensor missing from the scan may simply be one you added before.

If the dialog finds nothing at all, check that DMD2 has the Bluetooth permission in Android settings.