The reason the water flows when you place the hand shower down is because you now have a lower point of gravity which tells your water flow there is a dynamic movement hence water begins to flow. When the hand shower is in the holder (upright position) there is no pressure registering as the water cannot rise up the pipe which suggests you have a gravity fed system with a pump? If so the pump is not activating to start the water flow. You will always get round it by dropping the hand shower down as mentioned above, or you would need to have the pump changed for the correct one which would be either a negative or positive head. The difference being one pushes water, the other pulls water depending on where the pump is in relation to the bathroom.