Keith,
Power cycling the control only clears the trouble. It does not clear the actual condition. Since the device was missing before the power cycle, it will go missing again quickly. A transmission from the CO detector will reset the missing timer, but a power cycle will not.
To confirm it is communicating with the control, go to Wireless Transmitters and then select the specific Wireless CO detector. At the bottom of the details page there is a section called Testing. If the device has communicated with the control, there will be a signal strength shown. If it shows Unknown, then the system has not talked to it since the power cycle or since it entered the missing condition. If that is the case, check the device, as there could be an issue with it.
Also, let me know what firmware you are running on the control. We just released an update. We are also planning an app update early in January and are currently flushing out a few bugs. One notable item in the upcoming app update is the ability to update the onboard wireless RF9 board. We have been updating that as well, and it has improved trouble and signal handling. If you are interested, email me at michael.burrell@elkproducts.com with the email tied to your Apple or Android account and I can add you to the beta test for the new app. Just be aware it is beta at the moment, but there are a lot of nice features and improvements in it.
If that CO detector is not responding, use the same email address to work with me and we will get it swapped out.
Thanks again,
Mike