Help... When the bass string doesn't sound right!

I'm tring so hard to wrap my old brain around the different modes. Somedays I have that AH HA moment and the next day I seem to be confused all over again. DUH!Anyway, I love the ballad "Two Sisters" as sung by Dana McCall on her blog, and I have been trying to TAB it in Aeolian DAC. I have the melody tabbed on C and the A string sounds fine with it, but the bass string D does not sound right!Any help would be much appreciated.