I have been doing Partial SLDLs and they seem to burn my hammies nicely. Conversely, my training partner was feeling it more in his back, so we tried to see what was happening differently with our form.
Firstly, he is quite inflexible in his hamstrings and archilles tendons. In my SLDL, I was able to go below my kneecaps and he was trying to mimic that same depth. However the bar would travel away from his legs and he would flex from the waist. I was focussing on him keeping a proper concave in his lower back and wasn't paying attention to this, but when I noticed this it made sense that he was feeling it more as a back exercise than in his legs.
Then I noticed the bar would travel out from his legs at the point where his butt stopped moving backwards.
From what I've read, and how I feel is 'right', you start with the bar next to your thighs and then rock back on the heels, sticking your butt out as if trying to knock over a small child. As soon as your butt stops moving laterally, you should stop descending, as from this point you start flexing from the waist and changing the muscle groups affected. I believe there are more beneficial and safer exercises for the back eg. hyper-extension. All the while the bar should be following your thighs and should not move too far away from your legs.
Now my partner stops descending at just above his kneecap but feels the weight constantly in his hamstrings and can start progressively loading the bar more agressively than before. He still needs to work on stretching- but don't we all.
Again, the key was to keep the bar grazing the front of the legs and stopping the descent when your butt stops moving backwards.