HRM-Dual firmware update - doesn't happen

I have two of these sensors. Garmin says that there is no way to manually update the firmware and that it happens automatically via the device they are connected to. But it doesn't. Both connect seamlessly and reliably to my Edge 840, and function well. One has version 2.10, the other has 2.50.

I want the latest firmware in order to have them connect to my Strava phone app when I am using it for walking. The monitor with 2.50 will connect, the other one is not recognised by the phone or the app.  I have tried forgetting and reconnecting them; batteries are fine. Suggestions?

I am also interested to hear about any generic advice for updating firmware on sensors of all types. I have an RTL 515 radar and Rally power meter pedals - updates often don't happen easily, I typically get numerous prompts to update, which I accept, and nothing happens.

Thank you