boiler pressure not increasing?

There is a bolier at the local FC that has recently been subjected to froxen and leaking pipes. the leaks have akl been fixed but now when we try to get the pressure back up to at least 1bar the boiler is not registering even though I havbe opened up the inlet valve to top of the wtare pressure. I have had two plumbers look at the problem but they seem to think ists the prerssure from the mains water not being enough to increase the bolier pressue?? Any thoughts?