Have you try to use different iodine colour circle? At the moment I prefer to use purple.
I found this posts for you, might be a good starting point:
http://www.cdags.org/dagforum/topic.php?id=178#post-3080
http://www.cdags.org/dagforum/topic.php?id=259#post-3695
and
I guess, not sure about it, so I ask it: Is it possible to view easier the old dag, because of the oxidation of the surface?
http://daginhun.blogspot.com/
http://www.facebook.com/DagerrotipiaDaguerreotype