Hi Char

Well im guessing telling them to rest is out of the case?
But If you are massaging the hamstrings and stretching them try also stretching the Quadratus lumborum. It could also help to watch the players turning techniques as a few quite often turn their lower limbs before their upper body which can lead to back pain. Also have you tried functional testing of the back with the players lying on the couch trying to extend their back?
Another option would be to look at their Pelvic tilt anterier or posterior as this could give you an indication of what could be tight?

Good question and I hope some of these options help.
Cheers Rob