Radiator fan does not work

When car is at idle A/C does not work. A complete new fan system/motor was fitted a year ago. When I got the gas topped up before going to my own garage the A/C engineer told me the fan was not working correctly and he suspected it was the brushes. It has been back to my own garage twice now who took it to their A/C engineer who told them everything was fine. Apart from fitting a new belt they said nothing else was wrong. However, the problem reappeared with my wife yesterday and when she returned home the car was overheating and I discovered the fan was not working. When I tapped it gently with the shafts of a brush it started to work and the A/C was normal.