Mostly Harmless is not giving Orangered Notifications

[zants]

I've been using the extension for a few days but have not been receiving notifications when I receive a message on reddit.

I've tried:

• Unchecking then re-checking the 'Check for orangereds' in the Settings.
• Changing the values for 'Check every...' in the Settings.
• Leaving the messages unread for multiple hours, a full day, etc.
• Leaving a tab of reddit.com open for a few hours during which I receive a message.
• Leaving reddit.com closed for a few hours during while I receive a message.

Anything I should try next?

[zants]

An update. Three times since posting this I've heard the notification popping sound. A visual notification never actually appears, however. (And that's only been three times out of fifty or more orangereds I've had since posting this.)

(EDIT: Clarity)