I'll be honest, from your description I don't understand what you are trying to describe as the problem. With that I apologize if any of this is useless info.
Is your 12 gram being punctured? If not it could be a worn out spring, the RVA isn't screwed in enough or the puncture pin is broken.
The valve stem shouldn't stick. If that's happening I would look to see if it's bent, extremely dirty or if something foreign is jamming it up.
Sometimes the 12gram changer can be extremely difficult to remove under pressure. Sometimes you have to empty the 12 gram by shooting.
I hope that was helpful.