A second surgery or revision surgery may be needed. X-rays should be taken (if they haven't been already) to find out what the problem is. There could be a bone fracture or malalignment of the implant.
Sometimes the angle of the hip into the socket isn't enough to keep it from slipping out. In other cases the muscles that help stabilize the hip are weak, never healed, or even ruptured.
Rarely further repairs or revisions can't help or may make the patient worse. It's always a good idea to ask for a second opinion when a second surgery is needed. You may even want to look for an orthopedic surgeon who specializes in hip surgery and specifically hip revisions.