If you experience any problems with event notifications, please first go to settings - notifications. There check that notifications are set to ON and try with the TEST button.
The TEST button can already detect most of the common problems that lead to notifications being blocked.
If the TEST works fine, but you still do not get any notifications from your events, Android fails to notify aCalendar.
This is most likely caused by energy saving options specific to your phone manufacturer or from 3rd party apps.
The website Don't kill my app offers guidance for many manufacturer-specific settings that may interrupt Android calendar notifications from working properly. Here are deep links for Xiaomi, Huawei, Samsung, One Plus, and ASUS.