radiators not hot

After turning heating on, radiators take ages to heat up. Thermostat on about 25. They never get really hot, always able to touch for quite a while. Put two new double rads in lounge and dining room last summer thinking the single ones were not enought but, they always seem cool at the bottom,
never adequately heat up the room.
Boiler is about 5 years old, so assume this is not the problem, although it does get a bit noisy, banging etc. My daughter has a flat and you can\\\'t touch her rads as they are so hot!
Would be very grateful for any ideas what the problem may be?

sounds like a circulation problem that may also need flushing. what sort of boiler is it & what speed is the pump on.

When installing these new radiators, did you realize that the water is always going to take the shortest easiest route from the pump discharge to the pump suction, and that you'd probably need to install a gate valve on that shortest easiest route to prevent that from happening?
By closing a gate valve on that short easy route, you pinch off the flow that way and force more of that hot water to take the long scenic route through the new radiators you've added.