2 radiators not reaching full temperature?

I have just added a small extra radiator to a large living room. Both radiators now take a long time to heat up and for a long time the lower part of both radiators are coolish. Other radiators seem to be ok.
The new pipe length is about 6m for feed and return and have been installed with Hep20 to reduce heat losses. A full system bleed has been carried out.