As magical and idyllic as Harry Potter’s world often is, it is replete with allegories of modern society and injustices. The tyrannous nature of Voldemort (Ralph Fiennes) is easily comparable to Hitler, as a man leading a "master race" of which he himself is not a member. The conflicts between the rich elite and the working class permeate every novel and film, and economic and social issues are a significant part of defining the world’s characters. One of the chief lessons taught by Harry Potter’s stories is the value of tolerance and acceptance. As such, it is curious that the film series failed to mention squibs -- non-magical people born into wizarding families -- in any capacity.
Much ado is made about "muggles" in the world of Harry Potter. Muggles are non-magical people from whom the wizarding world hides their existence. Sometimes muggles produce magical offspring (such as Harry Potter mainstay Hermione Granger [Emma Watson], whose parents are distinctly non-magical). The “pureblood” elite, or those who stem from families with generations of magical blood, refer to Muggle-borns as “mudbloods,” a term essentially used as a racial slur within the wizarding world. Purebloods see mudbloods as lower-class citizens not deserving of their magical abilities.
The opposite of this is a squib, or a rare non-magical person born into a magical family. Squibs live a life of guilt and shame for not inheriting the powers of their lineage, and are equally seen as lower-stature citizens by the elite of the wizarding world. They are forced to live in a world of magic without the ability to participate. Their squib-ness becomes a big part of their personality and provides a lot of behavioral explanation and context. The Harry Potter films contain characters who are identified as squibs in the novels, such as Argus Filch (David Bradley) and Arabella Figg (Kathryn Hunter), but are not acknowledged as such in the movies.
Of course, the novels have the space and time to flesh out characters with substantially more depth than films. Characters who exist on the periphery of the stories only receive the depth they need to make their on-screen actions credible, regularly sacrificing complexity for narrative movement. But given the film universe’s attention to Harry Potter’s class structure and social commentary, the mention of squibs would have been easy to insert with a couple sentences of explanation resulting in an added layer of depth to the prominent theme.
Argus Filch is perhaps the most notable squib in the Harry Potter film universe. As the caretaker/janitor of Hogwarts, the films depict him as a grouchy, grumpy, miserable old man. That depiction is accurate, but the reasons behind it are never revealed. As an outcast member of the squib community, he is a cantankerous old guy because he spends his days within a school full of children brimming with magical aptitude that he can’t perform even on a basic level. The kids are an endless reminder of his own failures, and not only is he constantly in their presence, he is forced to clean up after them and maintain the building that builds their skills and further separates them from him.
In the novels, Filch emerges as a somewhat sympathetic character because of this. He is just as cold and unpleasant as in the movies, but we are given an understanding as to why. In the films, he’s a jerk just to be a jerk, and those with film-only knowledge would fail to ever understand the source of his curmudgeons nature. A brief explanation of squibhood at any point in the films, aligned with the knowledge that Filch is one, could have enhanced his character and added another layer of depth to the rich social world of Harry Potter.
As it turns out, a scene in Harry Potter and the Chamber of Secrets (2002) where Harry (Daniel Radcliffe) finds Filch's Kwikspell letter (a course that teaches magic) was filmed. The scene attempted to inject squibs into the series, but was not very good and would only have been understood by those familiar with the novels (it provided no explanation of what Kwikspell is and thus has no logical context). The scene was cut for time and squibs were never written in anywhere else, taking with it the series' entire reference of the world's rare breed of social outcasts.