I am assuming that if you have poor water flow then the water is roasting, or failing that then the water flow is so low that the element isn't even switching on. With a rating of nearly 10KW you should be able to get a good shower except maybe on the coldest of days, where the temperature of the incoming water is a lot lower than normal.
As its a new installation, there maybe some debris in the water circuit, try removing the shower head and let the hose hang down and run the shower for a minute or two to see if that clears out any dirt and thus increasing the flow.