Upstairs rads not getting hot?

Hi, just wondered if anyone could help. I've recently moved into a 4 bed house which didn't have any central heating so I had this installed a few months ago. All the rads downstairs get very hot but upstairs only a couple of rads get warm and the others are cold. I've bled them, balanced them but it seems to me that there is a blockage somewhere. Can this happen on a new system or has anyone else got any ideas?