They're holding all my retention over defects that aren't mine

At the end of the defects period, the second release often waits for a certificate, and the certificate waits for a defects list to be closed. That list may include other trades' work, damage caused after you left, wear rather than defects, or items added after the period ended. Holding your whole retention against one small item, or against items that are not yours, is common and rarely explained.

Your subcontract decides what the release waits for: a certificate under the main contract, your own defects being made good, or something else. It rarely allows an open-ended list that keeps growing. Left unchallenged, though, the list does keep growing, and your whole retention stays held against work that was never yours to fix.

Ask for the defects list exactly as it stands. Split it into four groups: your items, other trades' items, damage after you left, and items added after the period ended. Price the items that are genuinely yours, make them good or offer to, and set that figure against the retention held. That turns a general refusal into a specific question with a number on it.

Retention Claim does that split from the list, your records and the release wording, then writes the request for the balance, with the evidence attached. Retention Loss Prevention records the end of the defects period and the evidence each release needs from the day you sign. The list is then answered before it grows.