I have added some SONOFF home automation devices to my Echo. These are added to eWelink app first and then to Echo with the eWelink Smart Home Fan skill.
They always show up in the Amazon Alexa App but the Echo refuses to identify any devices until I deregister the Echo and then re-register via the Alexa app.
More annoyingly after a couple of days the Echo forgets all the devices. All the devices are still visible and working via the Alexa app, but the Echo says there are no devices.
The only way to get the Echo to recognise the devices is to de-register and re-register the Echo. Very frustrating.
As a side note, if I include the device in a 'Routine' on the app, then that still works even if the Echo has forgotten the device in the routine. Bizarre.
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Have you checked to see if your Echo device is up-to-date with the most recent software updates?
To do so,
You can cross-check your version with the versions listed in the following Help page:
Alexa Device Software Updates
I also recommend disabling and re-enabling the Alexa skill to see if the issue persists.
Let me know if any of the above resolves your issue.