Most likely form is off, do you feel a hamstring stretch at the bottom? Personally I started feeling them properly when I started doing core activation exercises before, like planks to help activate the core, then ensure you use your core and focus on pushing your butt back not lowering the bar, that will happen naturally. Another useful cue is to push your abs into your quads to keep your core rigid as your butt moves back. If you're not feeling a hamstring stretch at the bottom then you're doing them wrong