Increase the stretching from once a day to 2 times a day if not 3. You should hopefully start seeing a difference then.
I say this because I remember a physio buddy of mine telling me the same complaint and how he then fixed it by just stretching more frequently.

Beware that too much hamstring stretching can hurt your back - lots of lumbar flexion can cause disc displacement and cause back pain. Ensure you follow your hamstring stretches with some backward bends or mckenzie extensions / cobra stretch.