Try temporarily turning off some of the other hottest radiators. If afterwards, the radiator gets fully warm, then the problem is the system needs balancing.
Check the DIY projects section of this website for information about how to balance a heating system.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!