Reading your other thread on the bbs I see you have sprayed oil down the barrel, if even a single drop has dripped into the hammer housing it will behave the way you describe. The hammer needs to be completely dry. Oil will cause drag and this drag will not be the same every shot and will also change with temperature causing inconsistent lower power and poor shot accuracy.
My next step if that had happened would be to strip it down and put it back to how it needs to be and start again with your pellet testing process but this would invalidate the warranty and you don't want to do that if you suspect the gun has issues in case they are genuine manufacturing defect ones!
It is very easy for oil and debris to get in the hammer housing on the multishot bsa's as the bottom of the magazine bay is open and leads straight there. I have even seen pellets dropped in there and find their way to the hammer!
I don't think you have many options, either send it back and plead ignorance or sell it me cheap!
HFT, for people with only 2 buttucks!
VooDoo TwoTwo The next chapter.