UA award flights

so i am trying to find a flight form STT to IAH and i am not finding any. there were bunch of those flights in a few weeks back. is this common with UA ? do i need to call them to book a flight or is what i find online ( via ua book with awards ) is what they truely have?

i've seen it before where i find more flights a few weeks later so i guess as those plans and schedules are done they put them on there...do i need to waite few weeks to see more flights or should i find an alternate way to book my one way to IAH..

thanks for al your inputs !