I've given root beer my first go a couple of weeks ago. I did use yeast, rather than force carb. I know there was some fermentation because there's clearly more yeast/trub in the bottom than it started with.
However, no carbination.
I don't have CO2 on hand to force carb, and I will check when I get home to see if the extract had preservatives, but I suspect the culprit is a leaky cap on the 2 liter bottle. Is there anyway to check for a leak?