The key to a thermosiphon system lies in the word siphon. Unlike a pump system the coolant must be maintained at a level above the upper hose and be free of trapped air. If these conditions are not met circulation will not take place.
regards,
Phil
And a good flow of cooling air through the Radiator, as the thermo syphonic effect needs a temp difference to 'loose' heat as liquid falls down through the radiator tubes, this is why many old cars boil in standing traffic
Sometimes the easiest solution is to add a radiator fan, which you can simply switch on when you feel the need
