She needs a coach that's not her dad. Plain and simple. She's gone as far as she can go with daddy. She keeps with him, and she is done as the rest of her prime years will be flushed down the toilet with early round losses to low-ranked players.
Almost 2 years later since her breakthrough (which she hasn't backed up since), I don't know if I'd still call it a slump. It's more likely that this is closer to her normal level. Expect her ranking to keep fluctuating between 15-40 for the rest of her career.