Your developer client may have told you they have already shared a document via 'Document Sharing', but you did not receive anything. In that case, it could be highly possible that your developer client has uploaded the document to 'Document Sharing' but forgot to click "Share". You may ask your developer client to check again.
If they have shared a document with you, it will look like the image below, where you can find documents in 'Shared with Me'.
Alternately, You can also share a document back to the developer in 'Share with Developer.'