Can gluteal implants placed under the muscle move out of place and what may be the reasons this would happen?

I have implants almost a year ago but one of them has given me problems why it feels and looks higher than the other apart you can touch the edges, a doctor told me that this is why one of my implants is above the muscle! How could this happen to me if they were supposed to be put under me, the implants can get out of the muscle? Why this would happen