If no water comes out of some of the radiators, the pressure is too low in the system to push water into the radiators. There is usually a fill valve to the system. If there is one, and the valve to the system is open, the valve can be clogged, so it would need repair. If the valve is closed, open it and see if water enters the system. The pressure gauge should indicate some pressure around 12-15 psig. To go further than this requires the services of an experienced heating plumber.