Fall through the cracks

To be overlooked, ignored, or failed to be dealt with properly by a system or process. Refers to something or someone that doesn't receive proper attention or care due to gaps in oversight or responsibility.

The phrase draws from the literal image of small objects falling through gaps between floorboards or pavement stones where they become lost or unreachable. The metaphorical usage emerged in the mid-20th century, particularly in discussions about social services and bureaucratic systems.