Usually it's because the eggs in the mixture haven't cooked enough to thicken it. Lemon curd will thicken as it cools, so check when cool. If it's still runny when cooled, pop it back in the microwave for a couple of one minute bursts. A failsafe way to check your lemon curd is done, is to check the temperature with a digital thermometer. It should read between 80 °C / 176 °F and 84 °C / 183 °F