I had an award ticket issued by Avianca DTW-IAD-FRA-KRK in J and I purchased a coach ticket KRK-VIE-LWO issued and operated by OS.
The first two segments (DTW-IAD-FRA) of the award ticket were operated by UA and the last segment (FRA-KRK) by LH.
The UA agent refused to check the luggage to the final destination on flights operated by OS. There was an overnight layover in KRK but it was less than 24 hours.
This is the UA policy which I found online:
Effective for travel on and after March 1, 2015, when a customer has two separate tickets, United will though-check bags only when the secondary ticket is for travel on the following carriers:
United and United Express
Star Alliance partner airlines
With the new policy, a passenger’s baggage will be checked between the origin and destination points that are reflected on a single or conjunctive ticket.
If the traveler holds a second ticket on another airline beyond the destination of the first ticket, United will check the bag to the destination on the first ticket(s). In such situations, the traveler must collect their baggage on arrival at their first ticketed destination, and then re-check baggage with the next carrier for their continuing flight(s).
My understanding was that UA should have interlined my luggage to OS. Well, they didn't. I wrote a letter to UA and no answer so far.