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.
If the user has ever said "no" to downloading the server pack for your server, Minecraft will remember that. From what I recall watching a few youtube videos, they can remove the server from their client list, add it back, and it will then re-prompt them.