If they are using simplex and not a repeater, you would have to be right on the airport property to hear them.
There are several reasons you may not hear what you expect to hear:
1. They are using low power hand-helds.
2. They are licensed under business names you may not expect to see.
3. They are a user on a local trunked system licensed to the radio service provider, and the end users don't have an individual license. This is very common.
4. They use radios licensed under two way radio leasing companies.
5. They use DMR or NXDN trunked systems licensed to the radio service provider, and the end users don't have an individual license, and thus would be unlisted. This is very common here.
6. They may be licensed on the frequencies that you see, but don't use them; instead they have moved to #5 above.
Thanks for the info. It just appeared that both AA and United are licensed under their own name, based on the FCC info provided above, as is Delta, which I can hear. Each airline is different, I guess. I'll have to get closer to the terminal and see if they do use low power handhelds.