Have you checked all folders in your email inbox? Notifications might end up in the spam folder, so be sure to check that one out (and, if they did end up there, mark them as non-spam).
Another possibility that comes to mind is that the buyers have used some of the forbidden words (email, Skype, pay, Paypal…); messages with those words often get delayed until an admin checks them (I’m not sure whether the same happens with orders), and once they let them pass, you don’t always receive an email notification.
If none of this helps, you may want to wait for a few days (and check your Fiverr account very often, so you don’t miss something) and see whether the issue is resolved on its own, or contact CS. The issue isn’t site-wide, as far as I know (I’m receiving notifications as usual).