This just happened to me, too - three invoices were showing nothing in the "viewed" section (only that they had been sent). I sent out emails regarding these invoices, and the customers informed me that they had received and viewed the invoices. I need to know that I can depend on the "viewed" section to be accurate in my AR efforts - I look incompetent to my customers when I don't have accurate information, and I have better things to do with my time than chase things that don't need chasing.