If you're using the web or desktop app and aren't receiving notifications, you may need to enable them.  First navigate to Strategies and click either the Reminders or Timers strategy.  This will open a dialog:
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Click "Enable notifications" and you should be prompted in the top-left corner of your browser to enable notifications.  If not, click the info button next to the URL in your browser and click to enable notifications.

Here's how this looks in Chrome:
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Be sure that Notifications are set to Allow:

Here's how this looks in Firefox.

Here Notifications are blocked.  To remedy this, click the X next to Blocked:

In both cases you will need to reload the browser and click "Enable notifications" again to get started using Notifications.