How can i fix Hotpoint Aquarius electric shower that just dribbles?

We have just moved in to an oldish house and installed a new bathroom. The old bathroom had a new-looking Hotpoint Aquarius electric shower installed, so we re-used that. The builder fitted it but it just dribbles and does not shower. I know that the mains water supply is good and strong because that had been re-done and we have another shower that works perfectly well off the mains. I could not find this model anywhere on the net ?! Any help gratefully received.