Has it ever worked, or is this a new problem? Check that its plumbed into the mains supply, and not fed from the attic cold water cistern, and check any isolating valves are fully open. Also check any inlet filters for debris or scale accumulation.
Static mains pressure varies depending on your supplier and drops with every storey upwards. What area are you in? I have measured pressures as low as 0.6 bar which exclude instant electric showers. Have a plumber measure your static and working water pressure. If the static is ok (at peak times) but the working pressure is too low, you can upgrade your pipework to reduce the flow resistance, and maintain pressure at the shower when it's on. If your static pressure is too low (check the unit's minimum requirements) then you are going to have to argue with your water supplier if you are on the ground floor of a single property, or consider a booster set (expensive) if you are in flats. Do you have a hot water cylinder? Might well be cheaper to by a mixer shower + pump!
PlumbSpark, October 2009
