You need to be pushing your butt back a bit more, and bending your knees a bit more. At the moment, you don't have much bend in your knees, which means you can't generate much force through your hamstrings, and you're probably therefore pulling the weight up with your back more than anything.
It might help for you to lower the weight a little bit, slow your reps down, and really focus on pushing your hips back so that you can then use your hamstrings to pull yourself back to the starting position. Check out the video below and try to copy my form with your RDLs.
Thanks Scott =)) I think I get it now. Definitely will try it out on my next training day =))