Two radiators, one upstairs and one downstairs are cold when the rest of the radiators in the house are hot. The two radiators are turned on full. What can I do to make them work?

Steam or hot water? Do you know how the house is piped? Is there one big loop with water entering and then leaving each radiator before it goes to the next? Or is it a manifold where some radiators can draw heat while others stay still.

It is a hot water system, and I don't think it's in one big loop, so it must be a manifold.