Not sure another dip would do any better. A dip is to kill infections, bacteria or parasites. After two these should not be an issue. It could stress the coral making things worse. You could always do a water change to compensate for no skimmer. At this point all you may be able to do is keep it fed until it heals. This will take awhile. From experience if flesh is missing from the center ring area. It is not good, but you have nothing to loose so keep at it.