I'm doing something very wrong because my best is 345 at 190. Most recently I only anaged 330 at 200. How far apart are your feet and do you just lift straight up past your knees?
The key is to have 'open' hips. Some of this is down to stance, and some of it is down to flexibility.
Stand tsll, and do 5 jumping half squats. Allow your legs to relax in the air, and to find their preferred landing width for stability and to launch the next squat. At the end of the 5th squat, hold your position, look down and you will see your natural width and angle from the hips. Seek to replicate that when addressing the bar for DL's.
Also, work hard on soleus, glute, adductor and lower back stretches, and consider roller work on your ITB to free up your knee/hip alignment.
The technique itself, you may try from a slight elevation in a power rack to ensure that you can maintain proper alignment in the start position.
Remember that the focus is not so much to lift the bar upward, as it is to fire the hips forward- you need to lift from the glutes and hips right from word go, dont engage them after the bar has left the ground by back power alone.
Head up. Hips low. Back flat. Traps taught. Balance 'sat back' through the heels. Sensation like body is a coiled spring. Drive the hips and the bar will do what it must as the body unfolds.