I don't know what can be held back or when to release it
There is money still held at the end of the build, but the dates, reasons and next decisions are not clear.
What's happening?
Money held back at the end of a build is usually connected to the contract's payment and completion arrangements, not simply to a general sense that the job is unfinished. The relevant dates, certificates, payment notices and the wording of the contract help explain what has happened so far and what the project record says is still outstanding.
Set out the record in date order: payments made, sums held, the completion and defects milestones in the contract, and the items still open. Keep evidence of inspections, photographs and correspondence with that timeline. It will make the next conversation more specific without assuming what any contract term means or what either party is entitled to.
The solution
Bring the account, outstanding work and contract dates together before deciding what to ask about next.