Romie, I'm speaking as a student, not a trainer... :) I did take group lessons in past and never progressed much (I did take them in 3 different barns). I did a good progress in my privates though. So its definitely something common. May be bring student one by one in mid of the arena to work individually while others are trotting around.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."