Sometimes the USPS guys are too lazy to deliver all the mails which they are supposed to deliver on that date. It has happened a couple of times to me. It will show out of delivery, then around 6pm, it will automatically says couldn't be delivered because recipient wasn't home (despite the fact that I was at home the whole time.) It should show up the next day.

However, once a very important mail got lost. It was a priority 2 days, but the mail was not delivered after 1 month. Tracking was also not updated after it says out of delivery (so for about 28 days, the tracking just kept on saying out of delivery). Tried calling them: had to wait for about 45min just to talk to a representative. Couldn't take it anymore, so I actually went to USPS mail distribution warehouse (not the USPS mailing centers). The supervisor was nice, and finally received the mail. It was underneath some cabinet, according to the supervisor.

Best of luck!