Help - Montblanc Doue Classique Ballpoint Pen Blue Hour - Something broken

[john.perrin]

Help!   I have a Montblanc Meisterstuck Solitaire Doue Classique Ballpoint Pen Blue Hour and for some unknown reason why I press down on it the ink refill pushes back up into the barrel so you can't use it.

 

When taking it apart I think it's do with the little black part in the image.  It seems to be popping up when you tighten it or it's in the wrong position.

 

Can anyone help with what's wrong and how to fix?

 

Thank you so much john