What Causes Thin Stool Despite A Normal Colonoscopy?

I m a 51 year-old male, pharmacist, generally good health, with exception of Meniere s disease (since age 13; controlled), and somewhat chronic GI/bowel issues. My BMs are like ribbons and have been for several years now. Colonoscopies are negative; dx is enlarged prostate compressing rectum. PSA is normal. Started on finasteride 10mg/day 4 months ago - no change to date. Thoughts?