Just guessing here but if the cells produce glucose, and they have sufficient O2, they are also going to utilize it for growth and metabolism, so you are only going to get a likely low steady-state concentration of glucose. You would need to continuously monitor glucose oxidation using your kit over the course of your experiment to observe the cumulative glucose synthesized. This doesn't work well since the detection system (peroxidase + dye) spontaneously develops color/fluorescence over time. Also, free glucose in the cell is converted to G6P with the equilibrium for hexokinase for towards G6P. You can't add enzyme inhibitors to prevent glucose utilization since they are the same enzymes in the glycolytic pathway which form glucose while running backwards. If you're supplying the cells with plenty of lactate and keep the O2 low, they'll be running anaerobically so not utilizing the glucose and it's possible that your cells are converting the glucose to glycogen so you would need to include glycogen phosphorylase to break down any glycogen to glucose. We produce these kits, but you need to have a clear picture of what you want to measure.