This situation is typical as high-pressure gauge will show a reading of between 800-1000psi during whole CO2 cylinder lifespan. That is due that pressure of CO2 in the liquid state is 800-1000psi. Once you use all liquid CO2 and CO2 will be only in the gas state (cylinder almost empty) gauge will start drop. This is a point where you will need to source new cylinder or consider refilling your current bottle as you will have from few hours to few days of gas left; depending on your bubble rate.