After a period of successful use, my Amazon Alexa or Google Assistant cannot control the Meross device.

Have you ever renamed a device or added a new device?

If the answer is "Yes".
For Amazon Alexa users: Amazon Alexa does not support the automatic refresh of the device list. However, if you want the renamed/newly added device to take effect immediately. You can manually add a device through the Amazon Alexa app, or you can simply use the "discover my device" voice command.
For Google Assistant users: Google Assistant automatically refreshes its device list. You can unlink your Meross account, and then link it again through the Google Home app to manually start the device list refresh operation.

If the answer is "No".
Usually, this happens rarely. But when it happens. Please perform the setup step again.
1. Reset your Amazon Echo or Google Home devices, then set them up again.
2. Unlink the Meross account and then link it. If this method is still not feasible, there may be a problem with Amazon/Google, or Meross. Please send an email to support@meross.com. We will resolve it with the highest priority.