The researchers in this study found that well-trained powerlifters had deadlift sticking points around 95-degrees of hip flexion – this is typically where the mechanical advantage in the deadlift is the smallest. This means that you need to train movements like block pulls, rack pulls, and RDLs to improve strength in this position and increase your max. Bands and chains are another great option for improving strength in the top half of the lift.
Reference: Hales, M. E., Johnson, B. F., & Johnson, J. T. (2009). Kinematic analysis of the powerlifting style squat and the conventional deadlift during competition: is there a cross-over effect between lifts? The Journal of Strength & Conditioning Research, 23(9), 2574-2580.