Montblanc rollerball cap won't post properly

A little over six years ago I bought a Montblanc Classic Rollerball #163
(the slim version, in black resin with gold) that I put away and never
used until the other day.

When I took it out and started to write with it, as soon as the cap,
which I had posted, came into contact with my hand just slightly, it
immediately popped off the pen. I have tried this many times and always
get the same result.

I have owned a Parker and two Pelikan rollerballs, plus many Parker and
other brands of fountain pens, but never had this problem before.

The store I bought it from Write With Style, in San Francisco, is out of
business, so I called the San Francisco Montblanc store. The person who
answered the phone told me that a service bulletin had recently been
issued saying that Montblanc pens are not designed to be posted.
(Someone should alert whoever produces the advertising photos.)

She also said that she didn't know what could be done, if anything, to
fix my pen. If I wanted to send the pen in for service, there would be
no guarantee it could be fixed at all and it would cost a minimum of $70
even if no repairs were made.

Of course I guess I could use the pen without posting the cap, although
in many situations away from my desk, it is a nuisance. Also, the pen is
too short for my hand and doesn't feel as balanced that way.

I must not have tried this particular pen in the store or I would have
noticed this immediately, so evidently it doesn't occur with every pen.

Anyone ever heard of this happening? Perhaps someone knows of a simple
way of fixing it?

Charles