Vanilla-looking custom items can result from failures at several points. Start in the server console and check whether /iazip completed without a build error.
Try the exact player download URL from another network
Download the same ZIP address the client receives. A login page, file-sharing landing page or 404 points to a hosting problem. Replace the URL below and check that the downloaded file is a readable archive.
curl --fail --location --output pack.zip https://packs.example.net/server.zip
python -m zipfile -t pack.zip
If the download succeeds but the pack does not apply, read the first resource-pack error in the client latest.log. Check for pack.mcmeta and the expected assets layout at the archive root. Check the resource-pack preference on the saved server entry.
On a Velocity network, identify which backend sends the pack. Compare the first join with a backend switch. If several servers prompt the player separately, review the single-pack arrangement in the ItemsAdder proxy guide.