Fighting the Elric brothers in an early episode of the FMA: Brotherhood anime, Greed eventually hardens his entire body, making him impervious to damage aside from directly transmuting his armored skin.
Later in the show, Greed has multiple fights against foes that do not use alchemy, and thus he should have an enormous advantage over them. However, he never again hardens his entire body—he almost always armors his limbs, but his chest, neck and head always remain vulnerable—and these weak spots always end up exploited.
We see him use his full power again in episode 58 in a battle against some nameless mooks, right after a near-total defeat in an important duel where he only protected his arms.
Why does Greed never armor himself fully when it actually matters?