While a long-running, successful movie franchise is every studio's dream, the logistics of keeping a series going can be an absolute nightmare. While it might be easy to plan out five films in advance on paper, so much changes during the production of each individual project, with certain characters and stories resonating with fans while others don't, often necessitating course-corrections that can destroy well laid plans.
And that's only from a pure story perspective, as the realities of making movies often pose the biggest problems when it comes to keeping talent on board between sequels. There's a reason why Marvel locked their key cast down for unprecedented eight-movie contracts back in the early days of the MCU, as an actor's schedule (or salary) can act as major roadblocks for any franchise.
With all this in mind, it's no surprise that some properties spawned iconic characters and then had no idea what to do with them going forward. Whether they took an extended break, with a comeback being hyped up that amounted to little more than a cameo, or whether they were criminally brushed aside in favour of a terrible new, younger cast, there have been plenty of sequels that have fumbled the return of once-great characters.