Boiler water pressure?

I have a Ferroli SYS 10-23 central heating/hot water boiler with very low water pressure in the central heating system......as shown on the boiler hydrometer. I've tried raising the pressure using the 'pressure loop', both while the boileres working and not working, & there is no increase in pressure registering on the pressure guage. Does the pressure loop need to be open for an extended period before any increase in pressure will occur ? I've checked water flow through the loop and that seems fine. Can anyone offer any advice please ? Thank you.