The usual recourse is to call in and ask them to direct-sell the flight. Rather than searching and not finding the flight (or the availability), they make a request directly to the partner. This will tell them for sure whether or not there's space. Most agents are quite willing to do this. It takes a few seconds, they just need the flight number, date and times and the class of service.
Sometimes their systems just get out of sync. It's not really shenanigans as far as I can tell. I've even had to get Aeroplan to direct-sell an AC award one time that I found on the UA site but didn't show up on Aeroplan.com or when the phone agent searched
