blocked radiator pipes

Hi all 4 story house 8 rads. I drained down our radiator system yesterday to clean the heat exchanger. I drained the system using the drain valve on one of the bottom radiators. I topped the system up, bled all the radiators from top floor down. All the radiators work fine with the exception of...

System is oil-fired (Worcester Heatslave). Everything was fitted in 1995 when the house was built. The pipes feeding the radiators are 10mm, a mixture of copper and plastic. Some of the radiators barely get warm. (Rooms furthest from the boiler). For one room, I was able to pull up the...