I don't know. I never noticed that limitation, and have never heard anyone else mention it before, so apparently most people who use it don't need to do that. I just looked and they don't see to mention anything about that in their Help, except to say that some airlines only release 1 or 2 seats at a time anyway.

You may have to do it "the long way" if you need 3+ seats, ie, picking an airline in the alliance that Award Nexus uses and searching at that airline's site directly.

Any site that tries to work with a bunch of different airline sites may have to limit its options to only what every site they use supports, while if you go manually to every site, you have the downside of leaning a different user interface at each site, but each site's individual features will be available to you there, while they're unlikely to be found at an "aggregator" site. (The same goes for a lot of paid searches too, there are fine-tunings you can do at some airline sites on a paid search that aggregator sites don't offer.