adding to playlist on iphone disappears when synced to itunes

I'm having trouble diagnosing a small but nerve-wracking problem. I am trying to add a song to an existing playlist on my iphone 4. I go through the process of adding the song to the playlist, and every time I sync with itunes, the addition will not show in itunes and it disappears from the playlist on the iphone. With the iphone plugged into my imac, under 'summary' and 'options', I only have 'open itunes when this iphone is connected' checked. How do I get playlist changes/additions created on the phone to sync properly when I connect to itunes?