This is most probably caused by device settings. You can solve this by following the below steps. Please note that steps for different phone models (even though same manufacturer) may vary.
Xiao Mi
Go to settings > manage apps’ battery usage > choose PrestoMall and select “No restrictions”.
Go to settings > permissions > autostart > choose PrestoMall and turn the switch on.
Go to settings > app notifications > choose PrestoMall and turn “Priority” switch on.
Open PrestoMall > press the “recent apps” button (the square button or the three horizontal lines button on your phone) > find PrestoMall > pull down on it and lock it.
Oppo
Go to settings > advanced settings > battery manager > protected apps > choose PrestoMall > click “Protect”.
Go to settings > apps > advanced > ignore battery optimizations > choose PrestoMall . “Ignoring” here means allowing PrestoMall to run.
Go to settings > notification panel & status bar > notification center > choose PrestoMall > activate “Allow Notifications” and “Priority Display”.
Open PrestoMall > press on recent apps button (the square button or the three horizontal lines button on your phone) > pull down to PrestoMall app and lock it Go to recent apps overview > lock the app
Hua Wei
Go to settings > advanced settings > battery manager > protected apps > choose PrestoMall > click “Protect”.
Go to settings > apps > advanced > ignore battery optimizations > choose PrestoMall . “Ignoring” here means allowing PrestoMall to run.
Go to settings > notification panel & status bar > notification center > choose PrestoMall > activate “Allow Notifications” and “Priority Display”.
Restart your phone.
Galaxy S8
Go to settings > search PrestoMall > click into it.
Tap on the three dots at upper right corner > go to “special access”
Go to “optimize battery usage”.
Tap on the dropdown menu that says “apps not optimized” and switch to “all apps”
When all of your apps are shown, find PrestoMall > tap on the switch to turn it off
Ensure that your phone doesn’t automatically kill your app so that you can get notifications on time.