Maybe try heavier? like 1 - 3 reps? But anyway, if you got it off the ground then you will get it really soon. I remmeber trying to get 405 deadlift. One workout it got off the ground 2 inches, another workout, upto my knee, 2 weeks later got it to above knee, and then finally locked it out lol. I was actually tracking my deadlift progress by how much 405 got off the ground, which was obviously a bad idea. Don't worry, as long as there is progress than something is working.