People can't download the resource pack

Today I added a resource pack on my server (properly, via dropbox) and it all worked, at least for me. I can play it just fine on the server and it installs it. But, when my friends join they don't even get an option to install it (and it doesn't install). Is there any reason for this?

They have server resource packs enabled, and yes, one of them has denied the popup in the past on another Aternos server. They still cannot install the resource pack.

I'd also like to mention that one of my friends has used a custom resource pack on his server in the past, which showed the installation bar, but none of the textures changed.