Maybe check to see if all battery saving settings are turned off. Whichever WiFi network you're connected to, ensure that "Auto-Reconnect" is turned on. For the apps where you do not receive notifications, go into the App settings by long-pressing the app icon and tapping the "i" icon. Then go to battery and set the app to "Unrestricted" to see if it helps.
Btw, does this issue occur when you are on WiFi, Mobile Data, or both?