You are missing methyl salicylate which is responsible for most of the flavor of modern rootbeer. It is found in wintergreen herb, wintergreen essential oil and birch essential oil. Sometimes you can buy USP grade methyl salicylate at some drug stores. Usually, about 1 drop per gallon of methyl salicylate, wintergreen oil, or birch oil is sufficient. Essential oils should first be dissolved in a carrier such as alcohol to foster dissolution in water.
An online source for food grade wintergreen, ginger, and juniper berry essential oils (all useful in rootbeer) is:
http://www.lorannoils.com/Products.asp?CategoryID=2&SubColumnName=EssenOils
I suggest you start with just wintergreen, sasafrass and sasparilla and make small modifications on future batches.
If you like Dad's Root Beer, try adding a small amount of allspice. (Some people say they taste anise in Dad's but I taste allspice.) By the way, regular 2 liter bottles of Dad's Root Beer are apparently no longer available, but 1 liter bottles are still available in the Chicago area for $1.