The auto-download feature for maps built-into Quake III engine games is inadequate and obsolete, and it's best if servers disable it. It takes forever to download a map and most people won't wait.
Before going to a server running a non-standard map, download the map from
www.lucasfiles.com) ,
www.pcgamemods.com) , or
www.jk2files.com) with your broadband connection and install it in your game before joining the server.