Without wanting to state the obvious too much, this touches upon an important distinction for basic WGM automount share 101... Login item = the smb://server/sharename path, dock item = the /Volumes/sharename path. The dock icon works because the login item has already mounted a volume with the share name. Adding the smb:// path for the login item can be a bit fiddly as well as you have to browse for it, if worst comes to worst just link it to /Applications or whatever to create a login item, then edit the link path in the details tab.