Could be that all the heat has been used up by the time the circulating water gets to these two rads.
Shut off all the radiators in the property except the two cold ones - run the system and see what happens.
If the previously cold rads now heat up then it may be just a matter of balancing the radiators - to share the heat more evenly.
If they remain cold then check the valves (in & out) are fully open - feel the supply pipe to check the temperature - call in British Gas for a power flush at £600 a time! Or - take them off and flush them out yourself -individually.
This is merely my suggestion - I have no skills or experience in this area - others may well know better.
Good luck...
Peccavi, May 2013
