@Dee76 Hello,
Here is a related thread providing some troubleshooting suggestions for notification issue, please check Case 4: Cannot get any notifications only when phone is out of home Wi-Fi or on 3G/4G mobile data and see it works for you:
[Troubleshooting] Tapo Camera Notification Issues
https://community.tp-link.com/en/smart-home/forum/topic/271186
May I know the model of your phone and its IOS or Android version? Does the phone connect to outside Wi-Fi or using mobile data when it could not receive the notification, who is the SIM card provider at that time.
Please check if the motion detection message being pushed in Message Center (envelope) on Tapo app?