As you've probably guessed, your system is down on pressure, probably due to high water demands elsewhere on the street. Electric showers rely on a minimum base pressure for them to work and yours is now down to that minimum. The only thing that you can do is to get someone to check the filter in the shower water inlet (which should have been fitted during installation), and see that your house stopcock is fully on. Make sure that there are no other appliances using water in your house when you shower, and if all else fails, contact the water board who should be able to increase the pressure to your area.
John
April 2009