Then you don't have enough gas in the mag for more than a couple shots.
Greengas blowback systems are evolving...
May I ask what gun you're using that you've noticed cool-down with?
I've used a TONNE of GBB's and I only find cool-down to be present in poorly maintained and low-quality GBB's.
Use a TM or a KSC gun and you wont notice it at all.