Maybe because your fat grafts didn't "survive".
The survival of the fat will depend upon surgeons skills and will be the result of how the fat was harvested, how it was purified, and how and where it was injected.
When done successfully, the injected fat grows a new blood supply and begins to receive the nourishment it needs to survive from your body. When this happens, results can and often are permanent. However there can be a percentage of fat that does not take and this is why touch-ups are sometimes necessary.
The after photos were taken 18 months post op.