Sounds like a similar problem to the one I had in a previous house. It was a modern, well insulated, double glazed house with the stairs in the living room. On cold days it could take most of the day to heat up the living room even though the radiators were hot. On very cold days I had to use an...
I had similar problem with one of my radiators which took about 4 hours to get slightly warm. Turned out to be a sticking thermostatic valve. It might also be worth checking the lockshield valve.