Just a suggestion . . . when I first did stiff legged deads I really concentrated on keeping my hams straight and taking my lower back out of it, the same as if I was doing a static hamstring stretch, lowered the weight and really tried to feel the stretch in my hams, almost using the weight to increase/lengthen the stretch. Found that did the trick and after that, as I increased the weight, I always feel it.

It's hard to explain on paper, but once you get it you'll know exactly what I mean.

I just love stiff legged deadlifts. Perfect compliment to squats (on separate days for me)