is your effluent drip rate steady? if that gets blocked, then there is more backpressure in the reactor, which means the CO2 needs more pressure to be able to inject.
what's the tank-side pressure guage say? it should stay right at ~800 all the way up until the end. most of the CO2 is in liquid form in the tank, as long as there is still liquid the pressure will remain the same (as you let co2 gas out some of the liquid evaporates to replace it, maintaining the ~800psi). once you are out of liquid co2, the tank pressure will drop like a rock. the only way to measure how much co2 is in a tank is to weight it, which is why it's sold in 5#,10#,20#,... tanks.
if you are low on CO2, change it out now. most regulators seem to go crazy and free-flow once they get below a certain tank pressure.
