Brazilian Butt Lifts are almost entirely technique-dependent. The amount of fat loss has to do with how your surgeon harvests, processes, and injects the fat. Minimizing pressure to the area will help lessen the amount of fat loss. You cannot "feed" the fat. I repeat, you cannot drink ensure or eat a lot of calories in an attempt to make your butt grow. It is not how it works. If you are able to "feed" the fat to make it bigger, than the butt will certainly shrink as soon as you stop piling on the calories. Fat transfer is a very complex issue, which is why you need a board certified plastic surgeon who does at least 6-8 BBLs per week and can show you at least 100 before and after photos. You should be patient and speak to your surgeon about your concerns.
Usually there is a lot of swelling initially. But by about 6 weeks or so the majority of the swelling should be gone, and the fat that took should be stable.