Missing Invitation Cards

Two of my friends told me that they have not received my invitation card. I was surprised as i had posted one week ago. (Normally, it should not take more than 3 days to arrive)

So as to make sure my other friends received the invitation cards, i wrote emails and sending sms to check if they received. Some of my friends replied and they told me that they had recieved.

So what has happened to that two cards??? "scratch heads"

Hope that both of them will recieve the cards soon.