First of all, if they were previously working, you'd want to make sure it doesn't help to restart the phone. Sometimes notifications can temporarily stop working.

Next, it would be worthwhile to check in your iPhone's Settings, under Notifications, to make sure all notifications are enabled for RTM.

Also, you'll want to look in your Settings on the website, on the Reminders tab, to see if your iPhone is listed there and is enabled. If it already is, try disabling it, waiting for the confirmation at the top to show that reminders are turned off, and then enable it again.

While you're there, make sure the correct checkboxes are checked at the top (according to your preference).

Lastly, make sure you have tasks with due dates/times.

Hope this helps; post back if you continue to have problems!