The writers work so hard to avoid writing a Damsel in Distress that they forget to make the love interest her own character.
Name Space: Main
Page Type: Trope
You're creating a movie, and you need a love interest for your main character. You don't want to make her a weak Damsel-in-Distress type, so you make sure she's strong and independent. She doesn't need to change to be good enough for a man, the man needs to change to be good enough for her.
The problem? You go a bit too far. You're so busy not making her weak, you forget to give her any character flaws. Or weaknesses to overcome. Or lessons to learn. Or struggles of any sort. Or any storyline at all, outside of her romance with the main character.
In short, you forget to tell her story, or even make her a three-dimensional character. She only exists to play a specific role in a male character's story.
Examples include Captain Americaís Peggy, The Muppetís Miss Piggy and Mary, and The Artistís Peppy Miller.