I've done a lot of short trips for work out to hawaii on A fares (discounted first) for $900-1500. Typically out one morning and back the next day. In extreme cases out one morning and back that night.

What are the restrictions on these fares?

The reason I ask is I can't seem to get A-fares to come up on the UA web site for an upcoming trip (even though I can see plenty of A availability).

(My hunch is the problem is not actually the A fare restrictions, but is a UA web site/booking engine problem.)