There two basic kinds of story arcs: external and internal. Call them what you will. The external arc is visible and fully on-camera. This can be the must-save-the-world stuff, or it can be a mother planning the perfect birthday party. The internal arc is what happens inside a character. It often involves change, but it can also involve a refusal to change. It's thought and emotion.
(Note that traditionally an arc is a plotline that spans multiple episodes of something. I understand the word came into being in the '80s. My usage is more general. I don't use "storyline" because some writers associate that with the external plot only.)
However, the two kinds of arcs must affect each other, not run parallel to each other. I remember old Star Trek: the Next Generation episodes had this trend of running parallel arcs. In one, the ship is in peril or someone is going to die. The stakes are high. In the other arc, Lt. Commander Data tries to bond with his new kitten (and risks losing his humanity forever, or something.)
If an arc feels thin, light, or weak, fill it up with struggles and "trying." I've been doing this with The Superhero Murders outline lately (when time permits.) Once I have a grasp on a character's needs, wants, passions, social role and key relationships, I list their resources: that which they can call upon to achieve their goals. This helps me with the next step. I list their options. What can they try to achieve their goal? But I don't have them try just one thing, I have them try everything--so far as it makes sense for the character. I do this for everyone in the story, and soon I have so much material I have to begin making decisions about what can happen "on camera" and what stuff only I know about.
And since the characters often have conflicting goals and passions, this should naturally lead to rising pressure on each character. The actions of other character threaten that which they value. The cauldron boils until it explodes over the rim in a frothy climax (sorry, got a little excited there.) If that isn't happening, I've made some critical mistakes in the character development area and I need to step back.
So, to me, the middle should be the best part. It sounds like too many writers think of it as filler.
By way of example, let's consider Samantha, my main character's loyal best friend, a cheerleader. Even though she is Kim's best friend, she is terrified about the dangerous stuff her headstrong, reckless friend is getting into, so her goal is to get Kim back to doing more normal stuff--like shopping and dating. At the same time, she's also worried about pushing Kim further away and ruining a good friendship.
So while Kim is engaging in the external arc (and thus her own internal arc) Samantha runs through a list of options. Talk to Kim; express concern. That doesn't work. Try reverse sarcasm? Reverse psychology? Maybe, but they're likely to backfire. Talk to Kim's mother. Doesn't work. Talk to Kim's other friends. Talk to an authority figure. A serious talk with Kim. Plan an intervention. Plead with Kim. Pregnancy scare? Threaten suicide? Call the police and tell them everything!
The deeper Kim wades into her fight with the enemy, the more desperate Sam becomes. (If I've done my job, I've already established exactly why Sam feels this desperation and will go to extremes here.)
Some of these come out in the novel. Some she skips for her own reasons. But what about Sam's fear of losing Kim as a best friend? That's part of character development. She has fear as well as passion. That leads to dilemma and choice--crunchy stuff when it comes to internal arcs. She has to choose if she's willing to risk (key concept there) losing Kim if it means maybe keeping her off that dangerous quest. Is it worth it? Will it work? Is it her only option?
We'll see :)
Writer of speculative fiction. Current work in progress is an untitled dark fantasy novella.