It sounds as if the spray droplets were partially drying in the air before they landed on the radiator. This is quite common with spray can paints, and your description of holding the can nearer and getting a gloss on that area seems to confirm this. Causes can be low room air humidity, and warm room air temperature, or even a warm radiator. Try holding the can nearer, (about 18/20 cm) and to prevent runs, move the can more quickly as you spray. Practice makes perfect, as they say!