I've had numerous problems with UX GA's at DEN. Here's one a friend had -
He had a ticket on South African Airways from Johannesburg to IAD, connecting to United from IAD to DEN to BIL. All 3 BPs were issued by SA in JNB. Flew SA to IAD, cleared customs, rechecked bags with UA in IAD through to BIL, and flew to DEN.
Then, when he went to board UX from DEN to BIL, GA said his BP was not valid and he should have had a paper ticket!
He argued for almost 15 minutes, pointed out that he had already flown UA from IAD to DEN and that UA at IAD had checked his bags through to BIL. Finally, just as they were about to close the door, she let him on. Unbelievable.