Far too often, lifters set up to deadlift with their weight too far forward into their toes. One of my go-to cues for the setup is "armpits over the bar with maximal hamstring tension." In other words, most people need to get their weight back so they're in a better position to pull the bar off the floor.
One simple drill I like to use as a last resort is to place a box behind someone so they learn to get their weight back. The objective here isn't to sit onto the box and relax. Rather, it's to use it as a target to get the lifter more on top of the bar than in front of it.