Does the original code work? I'm talking about the one with the author and the year. Do they display correctly?
Assuming the original code worked, what does your code display?
Do you have access to the entire b:Source element? If it's a watered down version it might not have a b:Tag child. It would be strange, but it is possible as you only posted a real short piece of code which gives almost nothing to work with.
As you are clearly starting from another style, have you tried contacting the author of that style?
On a side note, displaying the brackets directly like that will cause problems when you want to combine multiple citations: [tag1, tag2, tag3]