When starting a series there is initially a lot of fertile ground for storytelling because the characters don't know each other very well and are still establishing an equilibrium. As a result, there is a lot of potential drama due to clashing personalities and the growing pains of the characters learning to understand each other. A very common character arc is having two or more characters initially starting off on a bad foot and learning to trust each other over the course of the story. For the audience, there is a vicarious thrill of the unknown in seeing how these two or more strangers bounce off each other and develop a relationship. This is the primary draw of virtually every buddy cop movie ever made, though it appears to a lesser degree in other works. Quite frequently the narrative payoff in the first book in a series is seeing these characters actually achieve some level of working relationship.
However, my question is what do you do for character development and drama in a series once the characters have settled into a personal dynamic and are no longer unfamiliar to one another? The relationship no longer has any conflict because they are familiar with each other's personality and quirks. It is possible to throw new characters into the mix to challenge them, but these relationships are often not as satisfying as the first entry in a series as the developing relationships are between a secondary and a primary character rather than between the primary characters. Other works have tried to keep the relationship between the lead characters unstable and ever-changing to maintain that feeling of the unknown. However, part of a friendship or romance is being able to trust one another, and if the relationship between two characters is constantly unpredictable and the two do not understand or cannot rely on one another it gives the impression the character never had a healthy relationship in the first place.
A good example of this is with the upcoming sequel to the recent Venom movie. Much of the drama for the original Venom movie came from the fear of the unknown, the symbiote was an unknown to Eddie and he didn't know if he could trust it. But in the trailer for the new movie the two are introduces as understanding one another and being familiar with one another's quirks. The movie seems to be trying to get around this by chucking Carnage at the protagonists and seeing what happens, rather than expanding Eddie and the symbiote's relationship.
Similarly, the Men In Black movies were based around the buddy-cop principle, albeit with the "cops" being alien police. However, by the end of the first movie J was no longer an inexperienced MIB agent and had established a rapport with K. This meant that the source of drama had evaporated and the sequels were unable to follow up on the success that the first was. They either tried to reverse the roles (as in the first half of Men in Black II) or do away with it (as in the second half of that movie), and got massively criticized for it as the "spark" of the original movie being gone.
My question is how do you continue to develop characters' relationships with one another in later entries of a series once they have settled into an established dynamic and there isn't a ready source of conflict and drama from the characters being unfamiliar with each other and learning to deal with each others quirks?