no more yeast doesnt equal more gas.
more sugar equals more gas
yeast are the living organism in homebrewed beer that convert sugar to alcohol and carbon dioxide via fermentation.
most (all) living things need energy. most use a chemical reaciton called oxidative phosphorylation to make glucose into atp (energy)
its very efficient. you can make massive amounts of atp from 1 molecule of glucose due to the amount of energy in the bonds of the glucose molecule.
fermentation is less efficeint. it only makes a few atp for every glucose molecule. but the best part of that is that one of the byprodicts is carbon dioxide.
that makes beer fizzy.
sugar plus yeast yield co2 plus alcohol.
but the yeast is not the limiting factor the sugar is. the yeast are like enzymes they can be used over and over again but they can only make as much CO@ or alcohol as there is sugar for them to burn. add more sugar you will get more gas ( and booze)
since its rootbeer you have plenty of sugar in there to make a lot of gas ( too much in fact. they will explode if not stopped somehow)
so you just need more time .
keep them at 72 degrees for a few days they will carb up rock hard and very gassy !
use at least one plastic pop bottle to test the firmness (carbonation level) of them.