If you schedule your manual notifications for a specific time and that time passes but they don't get sent out. Do they stay just "scheduled" or do they appear to get stuck in a "spooling" state?
If they stay "scheduled" double check that you are set with the correct timezone in your account settings. You can do that by logging in and going to Settings > Edit Profile. We don't automatically schedule ones that are too far in the past (to prevent people from accidentally scheduling it for the wrong day and causing it to go out instantly - which would be not what they hoped for)