I booked the following itinerary:

ORD -> LHR (UA928)
LHR -> ICN (OZ522)
ICN -> ORD (OZ236)

I was able to see all three legs on my account in United.com right after I booked. A few weeks later I could only see the UA flights and not the OZ. I have confirmed my reservations are there on the OZ website.

Anything that may have caused this? I suppose it won't cause a problem but I like being able to see my reservations in one place.