Can you start it alone in single player ? When you are trying to join your friends, have you all downloaded the latest version ? Did you download from separate links? (workshop and gamemaps). Should work but could be a glitch that is rare. Haven't heard anyone else who had this problem so there shouldn't be any issues with the download file itself. Did you try to uninstall and install back again ? Also you can try to disable addons, might have an addon that interferes with Lockdown causing it to bug out. Maybe verify game cache also if none of the above works.

I dont know how to tell if it's been downloaded or not but it did download when I clikced on "download add on" in l4d2 when I wanted to join my friends lobby to play with them.

It would have been the latest version surely?
Nope can't even see it showing anywhere when creating a game either! Despite it showing and fully ticked in my l4d2 add on main menu?? Tried disabling addons and verifying game caches

If you are fine playing by yourself you can always try to launch it through the console. The first map name is bt1. If it does not work by subscribing through the workshop you can also try to download the map from gamemaps site. It goes under the same name.

the best map <3

Similar problem here. I've subscribed to it, but It shows it as still needing to be downloaded. Haven't had this problem with any other maps.

does anyone got the issue prop_ragdoll backwards min/max, i dont have this problem in the past but now i catch this problem/ please help

having same right now, sub'd to map but it doesn't show up when creating server