What Causes Persistent Vaginal Discharge Despite Taking Metronidazole And Doxycycline?

Hi, my sis (24 - 30 years old) has been having these terrible vaginal discharge for almost 2 years now. I need your help in diagnosing the condition. Send the response to this email YYYY@YYYY Here are the details. She has been having waterly then thick whitish smelly discharge. She does not feel any pain, even when urinating. She is HIV non-reactive (done the tests myself so many times). It has always been treated like an infection, BV (usually Genta injection, metronidazole high dose and Doxycycline 7 days course). She used to be better 2 to 3 weeks then it would always recur (waterly then thick whitish discharge and smells). It has been like that for so long and it is disappointing now. Now she just finished antibiotic therapy a week ago (Metro, genta and erythromycin) but 5 days after the discharge returned. Now it seems like they return quickly. There is no blood in discharges. No pain no itching. She has done cervical cancer screening for over 5 times and all results are okey. She has visited a lot of gynecologists to no avail. Please help. Assist in diagnosis and possible treatment.