There is a limit on number of items. 50 subs I think.
The maps download into your Steam-Steamapps-common-killingfloor-map folder

When you sub they should download, but often you need to start the game and start a round and play. Go back to menu and see if downloads are shown upper right <white text>

After maps are downloaded, shut game down and restart... the map list is generated on start up so new maps won't show up on the map list in game until it is restarted.

If you are still having issues you can try to unsub from everything and resub to things one at a time... sometimes a lot of things at one time clog up the system, unsubbing then resubbing one at a time seems to help in those cases.

Many people, myself included, add a download link to the file in case anyone has issues with workshop. Just download the map manuallly and put it in your map folder mentioned above.

Remember also KFO maps only show up in the Objective game mode, Gun Game maps only show up in that game mode.

tl;dr if the map is in your Map folder it should be seen by the game when it starts. If you have to, place the map in there manually.