Danny's inconsistent immunity to Ember's music

In Ember's first appearance, Danny is notably one of the few people who hears Ember's music and is not affected by it. Sam speculates that him being half-ghost has something to do with it, which might be an acceptable explanation. However, later that same episode, Danny succumbs to Ember mind controlling him to be obsessively in love with Sam. Why would his ghost powers give him immunity to one but not the other? And if it's not his ghost powers that gave him immunity, what did and why didn't it work on Ember's "love song"?