Uh, what? It isn't expecting "United States of America" as the place of birth, but the state if USA, the country if not USA. I know that "US" is a code option in that field, but I've only ever used the state. The 2 letter code isn't 100% matched up to the postal code (NB is Nebraska for example, because NE is the Netherlands.)
If you just enter the first letter, it should show a list of options that match that (hence how you will notice NB for Nebraska....). The help guide calls it "predictive text."
The one that I had the hardest time figuring out was "Samoa". Not "American Samoa", that's easy, but technically "Samoa" is "Western Samoa" to the FBI, so you find it under that. and just typing in "Samoa" doesn't find anything.....
Edit to add: I'm guessing you have a "Researching" status because you entered "US" and not the state of birth for the customer. Longest "Researching" status I've had is 35 minutes (then they went on delay, thanks for wasting our time, a-holes!). Most checks are instant (New status box the second I hit submit.). The blue Researching status as noted is typically anywhere from 2 to 40 minutes, with the averaging being about 10. Instant responses mean there is nothing negative in the database that "hits", or it directly matches up to a Denied status (no human interaction.) Researching status means it's been kicked up to an examiner (actual human) for further review.