1. Just let it sit out a little longer. Since you're using PET, you can squeeze the bottles to check pressure. Even now, you could take the bottle out of the fridge and let it ferment some more.
2. More extract means more flavor. I'll make 3 gallons of root beer using a "4-gallon" bottle of extract and reduce the sugar by 1/4 so it isn't too sweet. Birch beer, I only make 2 gallons.
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