Realistically, there are only two plausible explanations. One explanation is that more product is needed to meet your expectations. A second possibility is that the filler isn't being placed into the correct anatomic area to achieve the look you desire. Sometimes the injector needs to consider not only the size and shape of your lips, but also the relationship of your lips to your teeth and bone structures.
Best of luck
Hyaluronic acid fillers such as Juvederm and Restylane are popular for lip enhancement since experienced injectors can achieve soft, natural-looking lips. After your next treatment, return to your injector two weeks later to see if you require more filler. Photos taken before your treatment would be helpful in monitoring your results.
It is unlikely that typical HA fillers would be metabolized this quickly, which may indicate the need for more initial correction. You have the option of trying other fillers, of which there are several, but stick to the HA (hylauronic acid) family, as others can cause deformities such as lumps or granulomatous reactions, visibilty, etc.
You could consider using an alternate product. Juvederm makes many different products you could try, as well as Teosyl. If you have been to various injectors and always experienced the same results it is difficult to blame it on technique. Perhaps attempt administering more product.
All the best,
Silicone injections are not a wise option. If Juvederm is providing you the results elsewhere but not your lips, then you may have an objective for fuller lips than you are communicating to your injectors. A picture from a magazine of lips that you desire may be helpful in communicating your goals.
There is no reason it should work on other parts of your face and not your lips. My assumption is you like them BIG and when the swelling goes down, it's simply not enough product in there for you. Most people need 1 or maybe 2 syringes for nice, plump lips. But for some people, they need more syringes because they want bigger volume. You are not metabolizing it in 24 hours - impossible. And DO NOT get silicone in your lips under any circumstances.
"This answer has been solicited without seeing this patient and
cannot be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."
I would definitely not suggest silicone injections to your lips. Silicone can cause granulomas which will, ultimately, be much worse than small lips. How much filler are you getting? Assuming you are going to a qualified plastic surgeon, you may just need more!