Sometimes it's just a matter of them stitching codeshares that are difficult to do yourself, or that an airline is unwilling to do.

Once an OTA found me a significantly discounted all American Airlines metal fare that I could only briefly find on iberia.com, which ticketed it with a series of BA and Finnair codeshares.

The flights never touched Spanish, UK or Finnish soil. Only Canada-->US--->France.

¯\_(ツ)_/¯

It disappeared from iberia.com so I quickly booked it via the OTA (hop2 by way of skiplagged, even though ti wasn't a skiplag fare) and it all worked out.

Not AA, Finn, BA, nor Google Flights, nor Skyscanner, nor expedia could find it. Just when I thought I checked enough OTAs, a 4th one finds me something the rest couldn't.