Typically when the story "drags on".
The plot isn't moving with the same excitement that it started out with, or the hero's goal is no longer tangible.
I'll use Skip Beat as an example:
The story started with the hero wanting to get into show biz with the goal of overtaking her ex in popularity. The goal is already intangible as the reader will never really know when or how that may be achieved, however the author has a strong start by telling the reader the goals the hero must achieve first.
- Getting into an agency
- Getting a role in literally any production
- Getting a lead role in a drama
And here is where it fell to pieces. Our hero nabs a leading role faster than her character development was finished. Now what? All there is to do is to keep taking acting jobs until the ellusive "goal" is achieved. Somewhere along the way, being popular stopped being her goal, and all the focus just went to her character development. She's not working towards anything, just working while growing as a person along the way.
The moment she stopped caring about being #1 should have been when the series ended. But the author kept dragging it on because they didn't properly plan the pacing of the story before starting. It's been running for almost 20 years and the main love interest confessed his feelings THIS YEAR. WHEN IS THIS SERIES GOING TO END?!
Y'all... Have a plan before starting your comic or novel. The successful long running series don't have a problem with dragging because the authors wrote concrete goals for every step of the way.
Intangible goals = bored readers.