Why does my lip filler disappear after 3 weeks? (photo)

I had 1ml of restalyne injected and it looked great but 3 weeks after it had almost entirely faded. My doctor agreed with this and gave me a complimentary top up of 0.4ml. Again, they looked great for three weeks but my lips are back to their starting point. There does appear to be a very slight difference but I expected 1.4ml to have more effect. My friends and family agree they see no difference.