Why is this my luck?

We mailed out our wedding invites last Saturday. Everyone that we can think of has gotten our invitations, including out of state people...except for my FMIL and FFIL (who live in MD). I'm about to because I get a status update each day on the missing invite. How can it be possible that of all invites they don't get theres? And I know I put the right address. UGH!