As I understand it, the arrows (w/ iron heads) would only penetrate steel armor if they hit it at a vulnerable point such as the joints between pieces. The headshots I imagine would be hitting unfortunate soldiers who raised their visors for a better view, since the visor hampered both visibility and breathing, but also protected a fairly vital part of the body. As Watchman said, the effect of the barrage should have had a serious detrimental effect on French morale, and combined w/ the incredibly muddy ground, the suction of said mud against plate surfaces, and crowd dynamics as the French body got squeezed into a steadily narrowing funnel, led to the debacle that followed.

Ajax