I believe we had this discussion a while ago (or maybe it was in shrub’s forum), anyway the most likely thing here is that the clients connect, they get redirected to the http download site and for some reason they do not finish the download.
These reasons are for example if the user cancels download by pressing escape, alternatively but I haven’t verified this yet is if they have cl_allowdownload set to 0 on their client, and a third option I can think of is if the webserver is “choked” and cannot deliver up the file for some reason.
Also the client may have connection problems or the map may change, but usually you get another error message in those cases (something with “ET 2.55 incorrect gametype” if I remember correctly from the top of my head).
If a majority of the players do get their maps downloaded correctly I wouldn’t worry about the server config 
(Easily tested on yourself, just rename any map on your client and connect to your server)