When it comes to static stretching, the frequency of exposure is important. You must stretch daily, even multiple times a day, for a good month to see significant results.

Try this: Measure your hamstring flexibility then embark on a regimen where you perform the closed chain hamstring stretch demonstrated in the video above. Hold the stretch for 15 seconds and do 3-5 sets every day (alternate sides each set). After 4 weeks, remeasure your hamstring flexibility. You should see a significant improvement.