Cloudveil Messenger needs background data permissions to get messages when you don't have the app open on the screen. If it doesn't have that access you may only receive your messages when you open the app. These settings may also vary by device but here is where they may be found on some devices.
Android 9
Go to Settings > Apps & Notifications > Cloudveil Messenger > App Data Usage.
Turn the Background data switch on to allow this app to use data when you don't have it open.
Android 8 (Samsung)
Go to Settings > Apps > Cloudveil Messenger > Mobile Data.
Turn on the switch by Allow background data usage.