This is a case where a bit of skeuomorphism (gasp) might have been useful. The momentary screen refresh flash on old-style LCD calculators helped indicate to the user that the input had been acknowledged. In the iOS calculator, the subtle darkening of the "=" button (which is also almost entirely concealed by the rest of the curled thumb in right-handed users) is just not enough feedback on its own.
729 divided by 27 equals 27.
That is why if you tap "=" its still displaying 27 and if you tap it again it divides by 27 and a 1 is displayed.
So the Calculator is working fine.