Sometimes, when a letter, numeral, or other part of the design is weak or missing, it is the result of "grease" or some other substance filling in the recesses of the design in the die, which prevents the metal of the coin from filling all of the recess. This is usually referred to as a "filled die" or "grease-filled die."
In the case of Lincoln cents, one of the highest parts of the design on the obverse, Lincoln's shoulder, is opposite the motto, E PLURIBUS UNUM, and the words "STATES OF" on the reverse. This sometimes resulted in insufficient metal to fill all of the letters, leaving some of them weak or missing.