Again, not knowing the soda stream bottles.... is it possible that they have an internal regulator that only allows so much pressure out? The way the trainer is "charged" it can only ever be equal to the pressure coming out of the "filler" bottle. If it is low pressure then you would only get a couple of shots before it gets week. If the bottle is empty or close to empty, same thing.
Coolfire tech support is pretty good. I would reach out to them.