Have you looked up/down stream of the hamstring for other things impacting the situation?
How often are you working on this?
Several shorter doses throughout the day are important especially in the beginning.
Were you injured?
What is your hydration like?
Episode 58: Weird Hamstring Problems and Simple Neurodynmaics
Pro Episode # 36 – Voodoo Floss Series # 3: The High Hamstring
High Hamstring Gnar, Trigger Points, and Up Stream/Down Stream
Episode 354: Make Your Own Super Floss/High Hamstring & Hip Gnar Gnar
Episode 272: TJ Murphy Edition and Hamstring Stiffness
Episode 104: Hammer Your High Hammy
Episode 82: Protect your scrotch or: Medial hamstring fun