Err check your map flags, see if there is anything for that particular map. If not, check the map server cmd and see if any errors are thrown.
It could also be something you diffed into the client (I know if you dont diff certain things the client bugs out). Like if you diff in to ignore sprite/act files and your grf is not updated to support different colors for hair/clothes the client wont crash but nothing will change.
Worst case scenario pm a contributor like judas or a forum mod with some screenshots and they should be able to tell you the problem
substep's post in Cannot Attack Monster and @Killmonster was marked as the answer