Give two or more indications that although Happy has a secure job he is no more successful than Biff.
This is revealed in Hap and Biff's conversation the night Biff returns home. Although Biff is the disappointing so, the one who has skipped out on every job & cannot settle down, Hap is unhappy himself. He has a low-level job, the kind that Biff can never hold onto. He does have chances at promotion, but it seems like he more often than not sabotages himself. He purposely ruins his chances to get ahead in life, much as Biff does.
During the brothers' revelatory discussion, Hap admits that he is lonely and bored, despite his outward signs of success. When Biff suggests they go west and run a farm together, Hap cannot bring himself to do it. He says he's "gotta show some of those pompous, self-important executives over there that Hap Loman can make the grade.” So there's obviously some kind of need to prove himself. He doesn't feel like he gets the respect or recognition he deserves.
Also, because of boredom and an empty sense of competition (perhaps residual feelings from his childhood attempts to live up to Biff), Happy has been conducting several affairs with the girlfriends of executives in his company. But even this brings him no pleasure. He acknowledges his dishonesty, & admits to taking bribes at work. So again, he is not living up to the "hard work" ethics that would actually get him ahead, but instead the kind of schmoozing and dealing that Willy taught them.