The website and the 1K Desk can't see it even though ITA, Google Flights, Priceline, AC.com and travel agents can all see it and they can plate on UA stock for me. They can see P space on same dates for the flight I'm looking at, but just not the date I want. Super frustrating because they only see D space for my date, which is like a $3k fare difference. I'm going with a travel agent for this complex booking but man, do I wish UA could see it either on the website or with the 1K desk. I am literally feeding them flight numbers and ITA fare construction but they just can't see the space. This is for AC 45 DEL-YVR on Aug 23. Not sure what gives. Seems like an IT issue or something or is it possible that AC is blocking UA from seeing that space for some reason?

Is this a UA codeshare (of an AC flight) fare class inventory issue. Not uncommon for the codeshare fare classes to different than the flight operators fare class inventory.

If others will ticket on UA stock, why not use them?

Nope, AC flight number, AC metal. No UA codeshare. (Yeah, I know codeshare fare classes are different...) Pretty much every other POS - LH.com, AC.com OTAs, etc. can see the AC space.

As to the second question, because agents are a pain to use compared to the website or the 1K desk - they don't work 24 hours so you lose space sometimes etc.! My itinerary is to complex apparently for BookWithMatrix because Priceline chokes on it. It does fine with 3 legs out of 4 but the 4th makes it break and ideally I'd like the 4th leg on the same ticket even though it wouldn't be the end of the world if I just split it...

If Google flights can see it why not just map it out there and then click-through to purchase on United.com?

United couldn't see the same P space on AC that Google Flights did. I needed UA ticket stock for RDM and PQM earnings. The OTAs through Google Flights would only plate it on LH ticket stock and Priceline through BookWithMatrix would choke because of the last YVR-SFO leg - it was fine if I just terminated at YVR. I finally got it ticketed through Propeller Travel which others have recommended here for complex ITA itineraries which you can't get ticketed through other channels. Fortunately for me (or my employer), the fare actually went down by $2k or something in this whole process.

so you had a UA segment in there? Otherwise UA won't ticket anyway. That UA segment could just mess up things and may not be able to combine with the AC P fare

Actually, now I have no UA operated segments - just UA coded LH operated segments. Initially I had a UA operated YVR-SFO segment in P (since changed to an AC YVR-SFO in P on the 787) which combined with the AC P fare without any problem. It's just that UA couldn't see the AC P space even though every other channel could. Bizarre that a TA could ticket something on UA stock which UA itself couldn't. I spoke to the 1K desk a couple of times - asked about a long sell etc. - but without any success.