Sometimes tomato plants produce good trusses of flowers but these wither and drop off without setting. This can be due to high or low temperatures, dryness at the roots or in the air.
Fruit setting occurs most freely in warm sunshine and in a moist atmosphere, so to aid pollination, spray the plants with water on sunny days around midday and then ventilate the greenhouse. Gently tapping or shaking the stems will encourage the flowers to shed their pollen.
If the plants are a dark, bluish-green and few flowers are being produced then they are probably suffering from an excess of nitrogen and this can be quickly corrected by feeding with a proprietary tomato food.