In My opinion, you need to figure out USCIS did not send or post office lost
it.
In my J-1 waiver case from 2006 to 2008, I experienced 3 times online status
were wrong,
First time, online status showed DOS sent sponsor view on 8/21/2006, but it
did not send.
Second time, online status showed DOS approved my case on 1/16/2007, but it
did not approve.
Third time, online status showed USCIS sent me receipt Number and A number
on 1/19/2007, but it did not send.
So contact USCIS by SR or infopass or