I wouldn't worry about it. In time with consistent training he will get it even with distractions.
Kaleb did not like any of the long sit/down/stand stays. He was so loud and vocal about his not wanting to do it. The trainer even tried to get him to stop and Kaleb was not having it. He was 4 to 5 months old then. He's 14 months now and he stays just fine with no "talk back".
Riley - 11/05/2012-8/30/2013 (Myasthenia Gravis)
He will always be in my heart.