It could also be that the problem are UA agents, who don't know how to properly endorse a ticket to another airline!
I had the same problem 4 times at different airports (BOS,IAH,ORD and DTW) this year (different trips

, endorsing to CO), everytime the issue was that UA agents hadn't done their part correctly and I had to run back to find an UA CSR to re-do the whole process.