Why are some gaps in Catullus’s poems impossible to repair?

The gaps in the poems of Catullus are impossible to repair primarily due to the precarious history of the text's survival. Because his work was not part of the standard school syllabus, it fell out of circulation from the end of the 2nd century until the 12th century.1

Our current knowledge of his poetry relies on a single manuscript discovered around 1300, which was copied twice and subsequently lost. The surviving body of work is derived from three later copies of that lost archetype. Over the centuries, incorrect transcription occurred, leading to instances where the text is mutilated or incomplete. Specifically, there are 14 instances of incorrect transcription that are considered beyond repair, and in 14 other cases, physical gaps—or lacunae—are visible in the text, eight of which involve the loss of one or more lines. Because the original source material is lost, scholars are often forced to rely on uncertain emendations to address these missing portions.1