You will most likely have lost the air charge in the expansion vessel, usually 0.5 to 1 bar. You will have to isolate the boiler from gas and electric, close heating and flow valves and drain the boiler. Use a car tyre pressure gauge and measure how much pressure is in the expansion vessel. If you need to, pump it up with car tyre pump make sure drain valve is still open. Close drain valve when re-pressurused, open flow and return valves, re-fill boiler, vent pump, open gas, turn on electric, try and run boiler. For the first few attempts only let boiler fire for a few seconds, make sure auto air valve is open. This usually works.
The only other common cause of boiler losing pressure regularly is if pressure relief valve is passing water.
Steve House, October 2010
