This was a problem because the two detectives were the stars of the story. They were always onscreen. Their relationship was the most important one in the story, and the audience experienced everything through their eyes. The two PIs helped to tell a story, but it wasn't their story. The screenplay, in other words, had failed to use up its two main characters.
Movies use up their characters.
TV shows don't. That's the rule. I hear you cry. But I promise to return to it before the end of this column. Since movies use up their characters, even secondary characters can exceed their plot function and possess fairly complicated psychology.