What could be causing my body not to retain lip fillers?

I’m hoping I can get some clarification. I have very thin lips… and over the last 10+ years I have had lip injections done. Different brands, different injectors but always the same issue. I get one syringe and for the first 2 weeks my lips look amazing which I understand is the swelling. After the 2 weeks my lips look completely normal again. I do drink a lot of water. I’m not sure what could be causing my body not to retain the fillers and I do want to do more areas but worried it won’t last.