Sorry to hear of this issue. Can you check the following?
1. Open iOS Settings > Notifications > Sorted³ and ensure "Allow Notifications" is turned on.
2. Try restarting your iOS device as sometimes iOS does get hung up on stuff.
3. Make sure your task or event does not have alert turned off. If you see a small bell icon with a line crossing it, then it means alert for that item is off. You can turn it back on by expanding the item, open the date picker, choose the "bell" icon at the bottom and turn on the switch for "Alert".
Let me know if any of the above resolves the issue for you. If not, we'll have to dig deeper.
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