Two tips -
1.) Try to bend your knees as little as possible throughout the ROM (of course, there should be a slight break in the knee). This technique places more stress on the hamstrings and let on the glutes.
2.) Concentrate on pushing your hips back as far as you can while keeping your chest and shoulders pronounced. This will stretch your hamstrings further.
Even if you don't "feel them", they definitely are being trained. I've done some wicked sets of RDL's and won't "feel" them much in the hamstrings, but they will get incredibly sore the next few days, so you know they are being recruited. You hamstrings have a very large capacity to do work compared to other muscles, it takes longer for them to feel fatigued.
You could also do some activation work beforehand, like a quick set of reverse hyper-extensions or something similar.