johnson and starley?

i have just moved into a flat and the vents in the 2 bedrooms dont warm up the room as effeciently as the rest of the flat
I switched on the johnson and starley unit for 7 hours which warmed up the rest of the flat with no problem but heat from the vents in the bedrooms trickles out very slowly-I am not sure whether i need to leave this on longer, or if theres a problem or if this is as good as it gets-in which case I am considering installing radiators! any tips would be appreciated