This page covers a few common issues known with watch apps. If this page doesn't answer your questions, you can contact me at wristinfo@gmail.com
Many people have reported that updating their iOS and watchOS did the trick. You might be using an outdated version. Please try updating first. Your watchOS version should be at least watchOS 9
After updating, if it's not available to install by the iPhone's "Watch" app (it should be at the very bottom of the screen), try to restart both of your devices. Keep in mind that bluetooth has to be active to install the app on your Watch.
Click here for instructions on updating your iOS: https://support.apple.com/en-us/HT204204
Click here for instructions on updating your watchOS: https://support.apple.com/en-us/HT204641
Please make sure to have at least one chat that is not archived in your Messenger. At the moment, WatchMessage only shows your inbox with active conversations. If you have chats in your inbox in Messenger, please check if you have logged in correctly using the iPhone app. If the login button leads you to a preview of your home feed, it is working correctly. If that doesn't work, try to restart both of your devices. The last thing you can try is to uninstall and reinstall the app.
If your problems aren't answered, please contact me, I am happy to help!