
Leg raises work your lower abs more than crunches, so I’d consider adding them if you haven’t already. Overall though it’s just time. As long as you’re increasing weight on ab lifts over time and either already have or are safely working towards a low enough body fat percentage, you’re making progress! Lower abs are always tougher since the body stores more fat there.
They do also work hip flexors, and either the hip flexors or lower abs can be worked harder depending on how you do them. You want to make sure you’re curling your pelvis up and toward your rib cage. If you’re using a captain’s chair, that means your glutes should lift off the chair a bit instead of being planted! It might also be easier to do knee raises first until you build up the strength and mind muscle connection for the exercise!