Why apparently verified Killing Floor Servers don't give achievements

Hi,



I'm wondering why some Killing Floor servers, fail to give any achievements at all, while others do.



I can appreciate whitelisted maps and perks-enabled etc, but this happens on a server by server basis - and I have all the "perk-enabled", "anti-cheat" etc options enabled, in the interface, when I search for servers



For example, this server http://ift.tt/1nrtDsI

never gives me an achievement for completing a map that I know I haven't got.



Yet another server, always does http://ift.tt/1nrtDsM



I've tried several times now, on various servers. I've completed Farm on suicide twice, but still don't have the achievement.



But if I go onto a server, I know to get them on, I always get the achievement when I've earned it... so it's something about the server.



How can I know what is causing this?