Hi I Have Had Post Nasal Drip For 6 Weeks

Hi I have had post nasal drip for 6 weeks now and although it has reduced I still cough all day from the white or clear nasal srio. Nothing seems to help other than Pseudo ephedrine based sinus meds and am on nasal cordico steroids 2 x a day and do saline rinse 2 x a day. I took antibiotics after the first week to get rid of minor sinusitis but the post nasal drip just won't go away. Also I had surgury to to straigten my septum and widen my nasal passages a few years ago. Any ideas why the post nasal drip won't go away?