Server Resource Packs don't work for everyone

I'm noticing that not every client receives the server resource pack from my server. Most do, but in many cases I do hear players saying the resource pack my server includes does not download to their clients.

Immediately I tell them to check "SERVER TEXTURES" in their video settings. In one case I watched the player on Teamviewer and confirmed Server Textures was already on, but the player never got prompted to download the pack.