Your boiler has lost water pressure and needs to be topped up to work at full capacity correctly.

DIY fix possible

Locate the external filling loop, usually a silver braided hose with one or two valves located underneath the boiler.Slowly open the valve(s) to allow mains water into the system until the pressure gauge reads between 1.0 and 1.5 bar.Close the valve(s) tightly and check for any leaks around the connection points.