Notifications are not working
If you are not receiving Trello notifications, there may be several causes.
You are not notified when you expect
Trello will notify you on actions that happen for a card you are assigned to or watching. This means that your icon or watching 'eye' icon is on the card.
Even if your email notifications are set to 'Instant', there's a slight delay from when a notification occurs in Trello and when the email is actually sent. If you read the notification in Trello before the email is sent, Trello will not send the corresponding email (due to the fact that you have in fact read the notification).
Due Date Notifications
You must be a member of card (or watching it) in order to receive Due Date notifications for that card. You may wish to watch the entire board. See Watching cards, lists, and boards.
If you set a due date on a card and you are a member of the card (or watching the card), you will receive a notification about the card twenty-four hours before it is due. Notifications for due dates only occur twenty-four hours before it is due and if you are not watching or not a member, it will not work.
Trello is sending you emails, but you're not getting them
You can test whether email notifications are going out correctly with this link: https://trello.com/test_notifications. If that's working, Trello is able to contact you via email. Still not receiving emails? In some instances Trello emails get marked as Spam or Junk and end up in your Spam or Junk folder. Marking the emails as not Spam or Junk should help.
See Not receiving confirmation emails or password reset emails for more tips on email troubleshooting.
You're not getting email notifications in a timely fashion
By default Trello is set to email you periodically with notifications you haven't dismissed in the web interface. To receive instant email notifications, go to https://trello.com/my/notifications and adjust your settings at the top of the page.
We want to be respectful of people's inboxes and attention, so that the Trello notifications match as closely as possible to the things they want to know about.
No option to enable desktop notifications
If you have previously denied desktop notifications in Chrome, Firefox, or Safari you may not see the option to enable desktop notifications in your Trello settings page. If this is the case then you can still enable Trello desktop notifications in Chrome and Firefox by selecting the lock icon in your browser's address bar and under Permissions changing the Notifications permissions to allow. In Safari you can enable desktop notifications by selecting "Safari" from the browser's header, then "Preferences", then "Notifications." In the notifications tab you will want to find trello.com in the list of websites and select "Allow."