Apparently the filling loop gets blocked. Quite common.
The way I resolved it was to put a hose on the drain cock and backfill.
Put the running hose in position on the drain cock and open the drain cock (or you will drain the system even more). The filling point knob under the boiler should be closed when you fill from the drain cock side.
Apparently you can clean and unblock the filling loop or replace I do not know how to do this.
BB Essex
August 2009