Radiators Are Cold?

Hi all, Just wondering if anyone can help... I have 2 radiators out of 8 which refuse to heat up at the bottom of the radiators. The tops are quite hot but bottoms cold. I have bled system several times but still no joy. They are standard radiators being ran from a Baxi Combi 105HE boiler. Also how can I find the last radiator in the system to bleed or does it not matter? Any help appreciated thanks.