In bodybuilding, the common use of muscle isolation leads to an increased rate of muscular adhesion. This can cause an increase in intramuscular scar tissue.
The presence of scar tissue is a precursor to decreased movement capacity and overall dysfunction. You must take your mobility and soft tissue quality into your own hands, literally.
They're one of the most difficult muscle groups to develop, but the cure to your underdeveloped hamstrings might lie in the answer to this question: "Which way do you point your toes during a leg curl?"