[fall] (through|between) the cracks

(of a person) to be completely ignored, usually in a system or process

The idiom "fall through the cracks" is a metaphorical expression that originated from the idea of something slipping or dropping through the gaps or openings in a structure, much like an object falling through the gaps between floorboards or the cracks in a pavement. In a figurative sense, it came to be used to describe situations where individuals or things are overlooked or neglected within a system, process, or set of rules. The phrase is used to convey the idea that despite efforts to create a comprehensive or foolproof system, there are inherent flaws or oversights that allow some elements to escape notice or attention.

Despite the new educational initiatives, some struggling students still managed to fall through the cracks, receiving insufficient support.
In the crowded healthcare system, it's not uncommon for patients with less visible conditions to fall between the cracks, leading to delayed diagnoses.
The social welfare program was designed to assist vulnerable populations, but unfortunately, some individuals still fall through the cracks due to bureaucratic inefficiencies.
The intricate legal procedures sometimes cause deserving asylum seekers to fall through the cracks, facing deportation without proper consideration of their cases.
Even with the company's strict hiring protocols, talented candidates occasionally fall through the cracks, resulting in missed opportunities for both the applicants and the organization.

(of something, particularly issues) to be completely overlooked

The origin of the idiom "fall through the cracks" is rooted in the metaphorical concept of something slipping or dropping through the gaps or openings in a structure. The phrase draws on the imagery of objects falling through the spaces between floorboards or the cracks in pavement. Over time, this literal idea evolved into a figurative expression used to describe instances where people or things are overlooked or neglected within a system, process, or set of rules.

Despite the thorough project management, a few crucial details managed to fall through the cracks, leading to unexpected challenges during the implementation phase.
The company's attempt to streamline its operations inadvertently caused some key customer feedback to fall between the cracks, resulting in missed opportunities for improvement.
The extensive review process aimed at catching errors in the manuscript, but a few typographical mistakes still fell through the cracks and made it to the final publication.
The comprehensive safety protocols in the laboratory were designed to prevent accidents, but occasionally, minor safety concerns could fall through the cracks if not addressed promptly.
Despite the meticulous planning for the event, logistical issues occasionally fall through the cracks, causing last-minute adjustments and challenges for the organizers.