In case of persistent infection, either u r irregular with the medication like inadequate dose or irregular timing. If ur compliance to drug is satisfactory, look for the cause of persistent infection...can be obstruction of urinary tract
, stone disease. Treat the cause and u can possibly get rid of infection. Get ultrasound
A 3rd possibility, u might be suffering infection from two different organisms or a resistant one. in that case u need to change antibiotics or add one.