It’s one of those things you just need to keep working at directly until you get it, but once you do, it will never leave.
One thing that can help is to be consciously aware to “roll” the barred index finger a bit. Instead of using the fleshy flat of the bottom of the finger, roll the hand a finger towards the headstock a little, so you’re using more of the SIDE of the index finger, the side closest to the thumb. The finger’s a little bonier there, and you get a little more leverage.