This Little Dick Goes to Market - S5-E14
Trivia: When Dick and Tommy walk through the Eversoll office, the door they open up has the name "Gary Parkinson." Gary was the Buckeye Fidelity insurance salesman from s05e03, "The Fifth Solomon." They also pass the office of "Ted Hughes." Ted also worked at Buckeye. Terry Hughes was the director of the episode.
Answer: Being promoted within the police department is usually a result of time on the job, combined with demonstration of talent and ability. When a character "cracks the big case" or "nabs the bad guy" this means that their superiors will take notice and possibly promote them if possible. In Don's case, he may already have the required years of service, but has yet to show sufficient ability.