To safeguard players' personal data, casinos delete uploaded documents regularly. This protects your private information from possible (although unlikely) data breaches.
Even though you've already uploaded your documents, you may have to submit them again, especially if some time has passed since your previous withdrawal.
PRO TIP: Keep photos of all your casino documents in an easily accessible folder on your computer or smartphone, or store them securely in a cloud. That way, you don't have to hunt for documents or take new pics when you need to upload them again.