Some tickets are not automatic and they do need to check that the availability is actually available (I was told this is due to using SHARES vs SABRE). Same thing happens with LO, I believe

They were not even entertaining AI for a while, as these tickets kept on failing. You might have to look for a different routing or get UA/AC miles and see if it works there

It most likely a combination of different systems that do not like talking to each other vs having a bad agent. Most of the female ConnectMiles agents are fantastic (Monica & Erika especially) and who can book exactly what you want (our most complicated so far was TYO-HKG-FRA-BOG-PTY in J). It takes a few minutes and be absolutely sure there is space available. AI might not play ball