1. Try to replace the water. Maybe you or somebody else put distilled water and forgot the Electrolyte.
2. Add more baking soda. But add just a little at a time, maybe 1/2 teaspoon, and make sure it completely dissolved in the water. NOTE: Do not let the jar get hot! You're producing steam, which will lower your mileage!
3. Add more jars. Connect them up electrically in pairs. That way, you have 6 volts (from a 12-volt battery) across each jar, which is plenty of voltage. The voltage beyond that only heats up the electrodes and the water anyway.
4. Are you sure it is too low? At idle, you're going to be producing tiny bubbles, like the fizzing of a soft drink. A lack of mileage increases may not be due to low HHO production.
5. If the voltage across each jar is less than 6 volts, you may need to rearrange your setup so you are getting around 6 volts per cell. Experiment with that, some users have success with 4-4.5 volts on each jar (3 Electrolyzers in series).
NOTE: even though theoretically, around 2 volts should be the ideal for each cell, in practice, it has been shown that about 6 volts is ideal for our type of system.