I had the same issue and received the below from Ecolink. It worked for me so give it a try.
There is an issue in the device where the clear command for CO is not sent if the smoke is detected at the same time. Have the customer try the test again, but cover the device during smoke (usually the 1st sound, 3 long beeps). Then un-cover the device during the CO sound (3 short beeps). This way the device only sees the CO sound and will send the restore soon after.