A few things I have been doing to help with this.
1. When warming up, do standing pullovers with a light band,, focusing on squeezing at the bottom. This helps my lats get going so they flex when I am deadlifting.
2. Work on your upper back. Do pulldowns with different attachments and lean back a little and squeeze your upper back on the contraction. 6-8 reps per set.
3. Focus on rotating your elbows towards your body. Once you grab the bar, imagine, rotating your hands out, so your elbows go in towards your torso, you will feel your lats contract.