Hey Matt!

Numerous reasons as to why you may be having hamstring pain but If you have been trying a lot of stretching, mobilizations, soft tissue work etc and still not finding relief there could potentially be a neurodynamic component to your issue. If you are finding there is relief from hamstring flossing that could be an indicator that this exercise is helping alleviate some of the neural tension. First you’ll want to look at how you are organizing the trunk and maintaining your positions. If you are overextended you may be putting the hamstrings in a less than desirable position so you’ll want to make sure you are in a neutral hollow position with the rib cage stacked. Next, there are several ways you can address some of the neural tension with some techniques such as tack and floss with sitting on a lacrosse ball underneath hamstrings as you extend and flex the knee and the banded hamstring flossing. Try incorporating some of the additional techniques in the video below and if they and see if you can start making progress on the issue.
https://thereadystate.com/2010/10/episode-58-weird-hamstring-problems-and-simple-neurodynmaics/

Cheers,
Mike