Usually they are obligated to accept one-time passes as United sells these themselves. Not sure why they would refuse entry (unless there is some unwritten rule about not admitting Chase or Partner provided passes when the club is full). The only time I have been rejected entry has been at SFO when I was directed to use another lounge when trying to use my Chase-granted pass. The club was full and there was a notice out on the desk. But this is dynamic and no way for a club to determine ahead of time.