Air cooled hoods are the most likely culprit. If you are using air cooled, use foil tape on all of the joints and seams on your reflectors and ducting. A fan drawing 300+ cfm through your room could also be sucking lots of your co2 out through a little leak.
maybe your plants r using the co2 more now. 40 minutes with no new co2 being added seems a little long. i may be wrong. so dont get angry and start yelling at me.
If you have no exhausts, the filters aren't being exhausted, and the air conditioner is hooked up correctly then I can't think what your problem is. Good luck.
A lot of dual ac's are not perfectly sealed. The easiest way to find the leak would be to run a smoke test and see where the smoke is escaping. You can also go down to almost any grocery store and buy dry ice for a few buck and then put it in water. If your ac is indeed leaking through small openings on the inside, you might be able to get some hvac tape in there to cover any leaking seams
Only thing you should do if your running sealed is get a controller that can seperate the co2 output when the cooling is happening yeah you may lose a little co2 if you get a nice size unit but. you want to remove stale co2 anyway. If you cant get a controller then you really shouldnt use co2 with an a/c unit. Myself i just ran into heating problems i bought the whynter 1200 btu dual hose unit. It comes on for 2 min and is off every 30 min or so but when its activated i have a sentinel chhc-4 which splits my co2 and cooling. i haqve noticed it takes it from 1550ppm to 1520ppm in the 2 min so it depends on whats better for you. also set you a/c on a platform higher up so it cools the top air more then the canopy air, Myself i have it on a rolling dolley from homedepot they cost 19 bucks and hold 500lbs.....