The works will never be certified complete
The standard keeps rising, the last percentage never gets released, and the account stays open indefinitely.
What's happening?
Every inspection finds something new, items closed months ago reappear in different words, and the standard being applied has quietly risen past the specification to whatever the inspector would prefer. Certification of completion - and everything hanging off it: retention, the end of certain obligations, the account itself - recedes at exactly the speed it is approached.
The counter is to make the target hold still in writing: the outstanding items fixed as a definitive list, each tied to the specification clause it answers to, closures evidenced and formally notified, and new items challenged against the contract's actual standard rather than absorbed.
Where certification is being withheld against a documented, closed list, the conversation changes character - and if it still doesn't move, that documented list is what any escalation will need. On rights and remedies, the escalation itself is a solicitor's territory.
The solution
Done defined in writing, evidenced, and held to.