help how to fix or sort radiator downstairs being cold?

Glowworm beatacom 30c.2 rads upstars get hot both sides fully.but one downstairs cold.drained systym fully.refilled.bled all rads in order 1 in bathroom .1st bedroom.2 bedroom.then front room last in line.heat in bathroom fully.both bedrooms fully.inlet pipe to rad front room hot.goes into f-room rad top half of rad is warm.rest cold.return pipe straight to boiler goes through hole in ceiling in room across back to bathroom straight to boiler nothing conected to that return flow pipe.boiler goes to 80degrees then cools down to 30 then heats up again.so no airlock or trapped in systyme.when rads switched off hot water gets to temp.switches of cools water to teped then warms up by boiler again.hav fitted 3 new rads a while ago.all ok.is pump suspected.or heat exchanger need flushing out.also how long should in line filter last before needs replacing.or would you suspect alas worst thing p.c.b.board.please advise very greatful.any idears its cold outside n just as bad inside m home.thanks nick the fusterated