When he was editor for the Superman books, Mike Carlin used to say that comic books were all second act.
Brief summary here: in Aristotle's Poetics, Aristotle says that a story should have a
beginning (act I), a middle (act II), and an end (act III). He lays out
some ground rules for what should constitute each kind of act. Act II
mostly gets ignored as "not the beginning and not the end". There are
also rules that are specific to the poetic performance medium he was
familiar with. All kinds of rules have developed since then, like the
one that says that Act II should be as long as Act I and Act III
combined, but basically? Act II is the middle.
And if you think about it, that's more or less true: Many (although not all) heroes have an origin, a first act of some kind, and very few heroes have an ending, a third act. That was one of the things that was amazing about The Dark Knight Returns, that it gave us an ending to our hero. I recently saw Superman Returns referred to as the Robin and Marian of superhero movies, and that is in some ways appropriate. The bulk of their adventures is the middle.
But I've also heard that the second act is the hardest for many writers to write in books, and we're voluntarily signing up for them?
Part of it is that there's some confusion about what the story is. Most of the fiction or screen-writing advice that I've seen treats stories as explorations of character through action: The characters learns or doesn't learn something and that knowledge or refusal to bend changes their world. Love stories are about giving something up so that you can take on something else--they're monkey-trap puzzles where the monkey realizes he has to let go of whatever's in the jar.
Since those days of the 1980s, we have seen a number of third acts, of ends to the superhero story. So let's riff off some of those to figure out how we can deal with the second act blahs.
Your campaign could have a story with all 3 acts. The story could be the whole campaign, or it might be just part of a campaign. It doesn't have to be the traditional origin-middle-ending so beloved of movies; it could be like a season of a TV series, where there's an overarching villain that brings out something specific in the heroes. The villain is a personification of a problem or a set of problems, and when the villains are defeated, the problems are defeated (for these characters).
Your campaign could have a change in personnel. One character leaves because his or her story is done; another character comes in. When a player feels his or her character is done, well, the character can retire or leave or be promoted.
You don't need to change. Look, not everyone wants to tell a story of that kind. It's not necessarily about how the character changes or makes a decision. What some people want is the very clever way to solve the puzzle that the bad guy represents. That's many stories right there. Another is the ability to impose order: something has gone wrong, and the character has to fix it. Orson Scott Card has these as the I and the E in his MICE quotient. Robin Laws suggests that this kind of hero is about acting in accordance with his fundamental principles. In that case, you can see where the character has compromised in some way, whether that was growth or a mistake, and push right there. Come up with a villain and an adventure that presses right on that decision: that it was wrong, or that the hero needs to do that more often in some areas that are clearly wrong by the hero's earlier choice. Then the heroes either change (one type of story) or return to their principles (another type).
1. You bury a jar and place a piece of bait in it. The trick is that the monkey can't pull out his hand while he's holding the bait. Unwilling to let go of the bait, the monkey just stands there, entranced by what he has instead of what he could have. In the beginning, the romance character has their current life, and is metaphorically holding on to the bait. One of the primary romance stories is about changing their minds.