How/why do people lose transferred fat after 3 months?

Dear Dunkindo,
fat viability depends a lot of surgeon technique. The fat has to be placed back into the buttocks as soon as possible so that it has a greater chance of survival. It has to be placed carefully so that not too much fat is placed in only one area.
Daniel Barrett, MDCertified, American Board of Plastic SurgeryMember, American Society of Plastic SurgeryMember, American Society of Aesthetic Plastic Surgery

Fat can actually be lost up to a year with stable weight, but most is lost in the first 6 months. In some areas, especially with scar tissue present or very dense fibrous fat, the fat grafts have difficulty surviving. Weight loss will shrink fat as well.

Excellent question, however the precise biomolecular, vascular, metabolic, and anatomic basis for successful fat grafting is unknown. Many techniques for fat transfer have been successful, but none are 100% successful. Similar approaches to fat transfer are successful in some quantities, distributions, and anatomic areas but fail when any of these variables changes, Every patient and circumstance is different.

Failure of one procedure does not necessarily predict failure of the same procedure repeated.