Why won't lip fillers fill the centre of my lip?

this is the shape of your lips.... if you fill this It will look sausage like. you can reverse all the fillers and start over which I would suggest If you want a different shape or contour. lip plumpers can also help (see link).

We all have different shapes to our lips. I would recommend that you look into the lip lift procedure for a more permanent solution. 

Dear pob_1997,
I perform a lot of lip injections with fillers such as juvederm and it works great. Many times I need to put more in one side than the other to attempt to even out the vermillion border (The red line of the lip). This helps provide more symmetry at times.
For patients looking for a more long lasting result, I recommend fat grafting. That is, using your own bodies fat to make your lips naturally look more shapely. The bonus is that the results last many many years.
In general, I avoid using permanent prosthesis because I find that the lips are such dynamic and sensitive structures, the end result is not natural.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery