I too have problems with the notifications. Sometimes they work as normal, sometimes they are late, sometimes they never come at all. I haven't found a fix. Some people install 3rd party calendar apps, like Jorte. That might be the only way to be sure to receive a notification. I have my calendar entries sent to pop up and email me (I set this via the Google Calendar website on my computer), so I just live with the notifications not be reliable. At least I get an email.