More and more, it was beginning to be borne in upon me what a particularly difficult chap Gussie was to help. He seemed to so marked an extent to lack snap and finish. With infinite toil, you manoeuvred him into a position where all he had to do was charge ahead, and he didn't charge ahead, but went off sideways, missing the objective completely.
The narrator reflects on the challenges of assisting Gussie, who struggles to take decisive action. Despite the narrator's efforts to guide him toward success, Gussie's lack of assertiveness becomes increasingly apparent. This makes it difficult to achieve the desired outcome.
The phrase "lack snap and finish" highlights Gussie's indecisiveness. Even when positioned to succeed, he tends to veer off course, ultimately failing to reach the goal. This ongoing pattern emphasizes the frustration of trying to help someone who cannot seem to act when it matters most.