A year and a half after BBL, I have a dent on the side of my butt. This is after my revision. What can I do? (Photo)

Most good plastic surgeons will do revisions without charging the patient.
The dent or uneven contour that you have may be due to uneven fat transfer or loss of grafted fat.
It may also be due to connective tissue fibers that hold the skin tight to the muscle layer.
These are the same connective tissue fibers that give some women cellulite.
Cellulite is caused by connective tissue fibers that connect the skin to the muscle layer and does not allow the skin to expand.
People who have these often will have flat or indented areas because the areas around them will not tolerate increased volume from the fat transfer.
the connective tissue will continue to pull the skin in towards the muscles 
To me it looks like there were some improvement from the revision.
Sometimes fat transfer needs to be done more than once to get the best results.
The key is to differentiate if this is a cellulite type condition or simply a lack of volume from insufficient fat transfer.
I suggest you go back to your plastic surgeon and asked for his or her suggestion of what to do and if the second revision is indicated.
Often people with connective tissue causing cellulitecan be detected before a Brazilian and flat areas can be predicted.
An experienced plastic surgeon should be able to anticipate who will get a good result and it will not.
I'm sorry to tell you but posting on real self will not fix your problem.
I suggest following up with your plastic surgeon or getting a second opinion from an in person consultation.
Best,
Mats Hagstrom M.D.