If a new character is added into an already existing story does that usually mean that the writers are running out of ideas ?

Yes, maybe, it depends?

It is generally considered bad structure if you introduce major characters in the second half of the story without any foreshadowing, with the keyword being “foreshadowing.”

For example, Jane Eyre introduced St. John Rivers (and his sisters) as a plot device to “save” Jane from starvation and homelessness and also provide an alternative for Jane so she could CHOOSE to go back to Rochester out of love instead of desperation. Because St. John Rivers was purely created to solve a plot problem, the guy (and his sisters) literally have zero depth in a book full of paper-thin characters.

Another famous example is Prof. Moriarty. In this case, the writer had not only run out of ideas but also hated his own creation and felt that he was chained to the grumpy detective. So, he introduced Moriarty with the sole purpose of killing off Sherlock Holmes. Then ACD realized writing full-length novels was a lot harder and a lot less lucrative, so he resurrected Sherlock, which undermined Moriarty’s character even more. As much as I love Sherlock Holmes, I think Moriarty is a piss-poor villain, and Sherlock deserves a better nemesis.

Great stories introduce their major stories early on, including the Big Bad. They might not have actual screen time (or page time), but they’re always there. Like Rebecca, the woman is dead when the story starts, but the story is so much about Rebecca that we don’t even know the name of the narrator. Many Whodunits also introduce every character, including the killer, but use red herrings and misdirect to make things interesting until the end, when the good detective declares, “The butler did it.”

So yeah, I think you’re right. If a story introduces new characters with a significant impact on the plot and then promptly disappears into the background, that’s probably because the writers write themselves into a corner. Instead of revising the story, they get lazy and throw in a Deus Ex Machina and call it a day.