I think i've found the problem.
Is not related to this plugin.
When you use sm_addban <steam_id>, the nickname isn't "uploaded" to sourcebans. In fact, is the sourcebans itself that searches for the player's nickname.
It searches in that link -
http://steamcommunity.com/profiles/X...XXXXXXX/?xml=1
However, and that's the problem, most of the times this xml can't be openned (why? I dunno, maybe because steam webserver is sh*tty?

). So, sourcebans cannot look for a valid username.
I tried to edit sourceban's .php files to search the nickname from the steam profile instead of that xml, but without success because I can't find a <div> to search the nickname. In fact, I found so many div's that the code picked a random one.
If anyone knows other place where sourcebans can pick the nickname, please fell free to tell me.
Sorry my bad english, but I think you can understand what i've writen.
Best regards