Ball valves control the water flow by using a rotating ball that has a hole through the middle of it. When the hole is lined up with the water flow the water passes straight through and when it is rotated at 90 degrees is blocks the flow.
It could be that what they are telling you is that there is space around the ball that is letting water leak regardless of the rotation of the ball. Have you tried replacing the valve? Although ball valves tend to last longer all
irrigation equipment wears out in the end so perhaps that is what is at fault.