Your messages could be missing because most certainly Mood is killed or paused by the Android System or a third party battery saver application (eg duraspeed) when the screen is switched off.
Android do this with to save battery life but prevent Mood from listening, receiving SMS/MMS or send MMS in background, but also from performing backups and lot of other things.
We invite you to:
- Re-check that there is no warning in Mood in the "Assistance" and "Notification"settings. In particular, allow Mood to run in the background and not be paused by Doze mode.
- Check the notifications can be displayed on the Lock Screen (Android settings then notifications menu, go to Lock Screen option and turn it on for Mood)
- Press on recent apps button, then long press on Mood and tap the lock icon
- Check you are in a spot where data is available
- Check you have enough mobile data or is it is activated
In order to allow Mood to run in the background, I also invite you to consult https://dontkillmyapp.com which provides all the necessary procedures for different manufacturers.
More radically there is also the complete deactivation of Doze on android with the command "adb shell dumpsys deviceidle disable" (via adb - not rooted) or to automate it on the phone with for example the Script Manager app from devwom (root required)