Collect the paperwork trade by trade as each one finishes

A subcontractor still on site, still owed money and still hoping for the next job will find their certificates in an afternoon. The same one six months later, paid off and busy elsewhere, treats the request as somebody else's problem. Collect each trade's closeout pack as they leave, and hold a little of their account against it where the contract lets you. Make the last payment wait on the paperwork.