Your iliotibial (IT) band is a long tissue that attaches at your knee and to a muscle called the tensor fascia latae (TFL) on your hip. Don't worry, there's no spelling quiz, but it's important to understand the anatomy part because often times the IT band tightens up when your TFL is overused.
In the absence of stability from your glute (butt) muscles, your body turns to the TFL, but it's a poor substitute. When the TFL becomes overly stressed, it pulls on that IT band. So no matter how much you stretch, it won't loosen up because your TFL keeps putting tension through the band.
You need to address the root of the problem: weak glutes.
Try gliding your IT band, TFL, quadriceps muscles and hamstrings over a foam roll to loosen them up, and strengthen your glutes with exercises like glute bridges and mini band walks. Include both moves in your regular workout routine and do them just prior to running.