I think there's some sort of catch you have to flick or a button you have to press or something for the gas reservoir to actually retain the gas to put into it.
It resets the firing pin or something, if you don't reset it then it's just like putting gas into a magazine whilst you hold down the gas release plate at the top.
As for them rolling out, you could try putting less bbs into the tubes? Or maybe stick some parcel tape over the end or something?
I've never used an airsoft grenade so this is all just stuff I've picked up from videos on youtube etc.