Loose Nibs

Hi again all,
I got a question, and yes, I have Da Book, so if it's there just point me to
the page and I will find other questions to ask. I have noticed on a couple
of older fps I have looked at that the nib is loose. It can be easily
shifted from side to side of the feed. When in the proper position these
pens can write very well but the nib tends to move from use. I am fairly
heavy handed and not much good for a light flex nib but my question is: Can
these nibs be seated back secure again so they stay stable in use?
R