Thank you for your question.
Even though it may seem that the marionette lines are a small area under the corners of the lips, they may actually represent a lot of area that may need to be filled in order to be fully corrected. Other factors that impact the appearance of these lines are the volume in the cheeks. The cheeks may be bulbous or the skin may cause a downward force on the marionette area that will make it seem as if any injection done would be of limited or no benefit.
Other factors to think about: whether or not the injections were done in the right plane, whether or not the right filler was used (sounds like good choices were made by your physician in your case), and whether or not enough was used. In your specific case, I would probably wait another 3 months and go back to your physician for more volume, if needed. You may want to ask / confirm that the injections are not being done too deeply, otherwise it will take a lot more volume and much more time to see the results you are looking for.