I think that symbolism is evident in a couple of points in the story. The strongest example of this would be at the end of the story, when Okeke receives the letter from Nene requesting her husband and children, his son and grandchildren, to visit. The moment he concludes the letter, Achebe uses the theory of correspondence between nature and emotions to bring out a symbolism that represents internal transition:
Very soon it began to rain, the first rain in the year. It came down in large sharp drops and was accompanied by the lightning and thunder which mark a change of season. Okeke was trying hard not to think of his two grandsons. But he knew he was now fighting a losing battle... His mind immediately returned to the children. How could he shut his door against them? By a curious mental process he imagined them standing, sad and forsaken, under the harsh angry weather—shut out from his house.
The symbolic connection between rain, its force, as well as the feelings of resentment and anger giving way to missed opportunity and sadness is powerful in bringing out Okeke's emotional state of being. In the ending, Achebe's use of symbolism helps to illuminate the desperation that exists in Okeke, a stark contrast to what he had been experiencing prior to it.