"This feels tight when I sprint" she said pointing to her hamstrings (the muscles at the back of her thighs).
And so she has been told over the years to stretch her hamstrings. And she does. Over and over and still it feels 'tight'.
So the question needs to be WHY is it 'tight' in the first place.
In this instance my clients sciatic nerve was being pulled at around 20 degrees flexion (as in lifting her leg up 20 degrees). This means that when she starts sprinting (and taking her leg past 20 degrees flexion) her body panics. As the sciatic nerve is being pulled at speed the body thinks "oh shit, we're going to tear this nerve in half" and so it sends the muscles around the leg into spasm to protect the nerve from stretching any further (Including the hamstrings).
Those hamstrings are trying their best for you not to injure the nerve. They are going into protective mode. And so when you stretch your hamstrings you're simply pulling on the nerve itself causing the body to panic and tense even more.
So what is the solution? Well one thing would be to stop stretching for a start and let the sciatic nerve calm down. Make sure you are warmed up and are well hydrated (as the nerve may be sticking to the layers around it). It's also worth noting that everyone is different so I can't actually say what will work for you personally (sorry I know that wasn't what you wanted to hear).
But what I would say is that if you have tried the same thing over and over and it's not working then stop banging your head against a brick wall. Start thinking outside the box and know that where the pain is the cause probably isn't.