I have seen this issue on hellsgamers server, when theres 3 or 4 theres either no zombie or it takes minutes for it to spawn, even if there is just one player they should become zombie, wierd.
As soon as more people join, thats if the other few havn't got bored and left, it works fine.
Yeah i have noticed this on the cssmc server to.
When i sometimes join my server and i'm alone or when there's a mapchange and i'm alone in the map, zombiemod isn't activated, but as soon as someone joins the game it load itself up. (not that it is a problem)
Well when i'm alone in a map i do not see the zombiemod msg's and i can buy snipers etc, but as soon as someone joins the zombiemod msg's apear when the round restarts.