I hear this same complaint a lot. Here is what you should do IMO:
Work on your form to the point where you "feel it" in your hams as much as possible. Then DON'T WORRY ABOUT WHERE YOU ARE "FEELING IT". Even if it feels like it's not working your hams, keep doing them. Here is a little secret. Even if you NEVER feel it in your hams, and it feels like all lower back--when your SLDL is 100 lbs heavier than it is now, your hams will be WAY bigger. Many people have the same problem, and decide leg curls and other isolation lifts are "better" because they "feel" the hams much more while they are doing them. You know why legs curls feel like they are really beating your hams up? because out of the 7 different muscles that make up the ham complex most leg curl variations work 2-3 of the SMALLER muscles. That makes it "feel" like they are really getting hammered--THEY ARE NOT!!!!
SLDL's work 5 out of the 7 muscles, and they work the "big" muscles that make up the hams. Much more "bang for your buck" wether you "feel it" there or not.
Iron Addict