I recently used ITA to price out a recent trip (BOS-SJC/OAK-SLC/SLC-BOS). Since ITA returned a cheap fare ($350 in K, T, & U fare classes) with two legs on NWA and one leg on DL, I emailed the itienary to my local AAA travel agent office to book it. I did not think I could get this complicated itienary booked on the two airlines respective websites.

My travel agent called me the next day. She said that she could not book those flights at $350. She was getting H & Q fare basis and a quote of $1000.

So I went to the websites and booked the trip myself. Sure enough, the lower fare buckets were available over the web.

Don't know why NW & DL would prevent my travel agent from booking the lower fare buckes.