Originally Posted by WineCountryUA

There are a number of things that may have happened
-- Award inventory is dynamic and what wasn't there 10 minutes can now be available and the reserse is equally true.
-- It may have been there but wasn't ranked at the top, the UA search engine ranks offerings based on a number factors (not publically known -- they probably include convenience factors {trip time, layover times, time of day, ...} and cost is there also -- but unclear how all this rolls together) -- you can force the lowest cost or least stops to the top (clicking on the column) to override UA's ranking
-- UA search engine "stops" after finding X results (200?) so potential solutions are missed
-- Lousy search
-- ....

For paid flights you can have the same issues and many use Matrix / Google Flights to a different / better set of results/ Not all search engines are created equal or work the same.