If your soda is not carbonated enough, there are several possible causes for this. The most likely one is that your drinks are being dispensed cold enough. Soda needs to be under 38 degrees or the carbonation goes away quickly. You can use a probe thermometer to check the temperature. A low tech way to do this is pour a clear soda like Sprite into a clear glass. If you have lots of bubbles to start with and 30 seconds later hardly any; you may have a temperature problem.
The cures for this are to make sure you have enough ice on your cold plate and that it is all in contact with it. Often when it is warm, a small “ice cave” forms above the cold plate which keeps it from picking up the cold of the ice. If you have a water cooled / ice bath soda machine (almost always a counter top model) you probably need to call the owner of the machine (often Coke and Pepsi) and have them adjust the thermostat.
Another possibility is that the syrup-soda mix (called brix) is out of adjustment. If a drink is too syrupy, it will taste sweet and flat. You’ll know that’s the case if some drinks are “flat” and others aren’t. If you take the valve cover off the valve that is dispensing the sweet drink, you can adjust the brix ratio with a small screwdriver. Look up the instructions online from the valve manufacturer first.
The least likely possibility is that your CO2 regulator is out of adjustment. The pressure at the top gauge should be between 90-120 psi. You can turn the screw in the center of the regulator clockwise to increase the pressure. Don’t go over 125 psi or you can trigger the pressure relief valve on the regulator and cause a CO2 leak.