V.S. Pritchett's father struggled with his businesses, including textiles, newspapers, stationary, and traveling sales (Wikipedia). During Pritchett's youth their family moved many times as his father failed at one or another business. "The Fly in the Ointment" is partly auto-biographical, and the character of the father in the story is similar to Pritchett's own.
After entering bankruptcy, the father is visited by his son Harold, a low-level college lecturer. The father is, to Harold's eyes, surprisingly relaxed about the loss of his business, which took up his entire life and was more important to him than family. A fly enters the room and distracts the father, who becomes agitated and attempts to swat it, then seems to grow exhausted and miserable. In an effort to cheer him up, Harold offers to lend him some money; the father becomes condescending and reverts to his old businessman personality.
Because of the personal ties to Pritchett's own life, the story has multiple levels; on one, it is about the many faces people wear to present themselves to family or to others. For most of Harold's life, his father's face was aloof and disconnected, and although he sees a spark of a new face, it is driven away by the old face:
...slowly the dreaming look went from his father's face... the little face suddenly became dominant within the outers folds of skin like a fox looking out of a hole of clay. He leaned forward brusquely... "Why didn't you tell me before you could raise money?"
(Pritchett, "A Fly..." scribd.com)
The story is also about the expectations of family to their children; the father "despises" Harold for settling for a low-paying job instead of working harder like he did. It is inferred that Harold is happy enough, but that was never a concern for the father, who simply wanted success and hoped his son would follow him.
Finally, the story is about the risk of offering help to family. Instead of continuing to accept his loss and refusing to burden his son, the father seems to be ready to take advantage of Harold, take his money -- likely at no interest or date of repayment -- and risk it on a business venture. Harold's genuinely sympathetic offer is taken as an ad hoc guarantee (see quote above) instead of in its charitable spirit.