Have you tried a measuring of the Pelvic angles to see if the patient has bilateral posterior rotation of the innominate bones such that the PSIS is inferior to the ASIS? Do the test with hip abduction for the most graphic stress on the innominate bones.
If there is bilateral posterior rotation, it may not be the hamstrings per sae.
It could be the adductor magnus that is in a guarding response or in excess tonus. Try releasing the adductor magnus. Part of the adductor magnus acts like a hamstring. I do not find many hamstring problems but lots of adductor magnus problems.
If only one side is posterior in rotation, then treat the adductor magnus on that side.
Hope that you find this helpful.
Best regards,
Neuromuscular.