Lip filler confusion. Is it possible to add volume to this flat area? (Photo)

I had 1 syringe of juviderm ultra injected into my lips. They are healed and I went back to injector and asked if there is a way to add volume to the flat area. She said no, that there is no way to add volume to the flat bottom lip. I’m confused because I thought if I purchased another syringe if maybe a different kind of filler that there would be a way to add fullness but I was told there is not.