There are a few things that could be happening here. During your warm up I would make sure you are getting your hip flexors loosened up by doing some light stretching (the couch stretch works great).
Once you’ve done that, you can implement a lot of different things. Some of the things I do are:
Single Leg Glute Bridges with 3 sec hold
Split Stance Lunges with rotation
Goblet Squats with Glute Squeeze
I do 1 set of 10 for each of the following, and then perform 15 light KB swings explosively and really focusing on squeezing my glutes. All this really gets my glutes firing and HR really going and ready to get under the bar and start squatting (or ready to start pulling). I think you’ll find this helps your training overall as you’ll be feeling really good once you get under some weight.
I would also highly advise you check out this article by David Allen:
There are plenty of things in that article that you could implement as well in your warm up or cool down to help work on this.
Out of all those options, I think you will be able to take away something and put it to good use.