The website was giving me a 5-hour connection in IAH, the connection I wanted is 2 hours, but it was not coming up in multi city search on the website and phone agents could not book it, either, even though it was available as a standalone flight.

But this morning I tried again and the connection I wanted was suddently bookable, so I was able to switch. Maybe this was a temporary glitch. In any case, phone agents were pretty firm about not being able to add the IAH-EWR flight as a connection even though it was available in XN as a standalone flight.