The walking lunge is a powerful quad-builder, but there's a better way to train this bad boy. Why does it fall short? Mainly the lack of constant tension.
When targeting a muscle, keeping some level of tension throughout a set is a necessity. But by alternating legs, there's about a 2-3 second "rest" period on each leg, which allows blood flow to rush out of the targeted muscles, taking away both muscular emphasis and metabolic stress. Do this instead:
Non-Alternating Walking Lunge
By leading with the same leg, you'll place more stress on the quads without ever relaxing the muscles fully between reps. If you're used to the alternating forward lunge, this will be a huge challenge and will cause a quicker, more painful training effect on the quads.
The increased accumulation of blood into the tissues creates an occlusion effect, which is maximized during constant tension movements. That's exactly what you want if you're trying to build muscle.