I agree with above stated reasons, and will add that the blank failure may have been aggravated by glue contamination inside the tube, causing added stress to the blank/tube assembly when the hardware was pressed in.
Often time when a failure occurs (in pen making or any endeavor), it is the result of multiple factors, in effect "ganging up" to cause the failure. So, when you experience a problem, it is good to not stop looking for the cause once you discover A SINGLE POSSIBLE CAUSE. Keep looking to see if additional factors had come into play. Then you can correct for all in future efforts.