One of the Hozelock 2802 watering system drippers has stopped working?

One of the water drippers in my Hozelock 2802 15 Pot Watering system has stopped working. This is watering a hanging basket. Another is also watering a hanging basket and the rest are watering tubs on the ground and these are are all working well.
The dripper worked initially with a poor flow but since then, nothing. We cleaned the stake to see if that was the problem but this has not worked. Can you please help?