Lack of hip extension
Your gluteus maximus muscle should be the prime extensor of the hip, but many people too heavily rely on their hamstrings. The problem is that the hamstrings are a two-joint muscle; besides having less of a mechanical advantage for hip extension, they also flex the knee, which shortens step length.
Fix it: Strengthen your glutes and increase hip extension with supine bridges. Lie on your back, knees bent, feet on the ground and then lift the pelvis while contracting the glutes.