Removed from credits after update

Hello,

I am writing this message to get some advice concerning a problem i have with my previous company.
I worked for them as lead designer and my responsibilities were to envision and designed games and prototypes.
Which i did with some iPhone games that were released during our collaboration.
Here comes the thing. Those games were updated recently and a friend brought to my attention that one of the new feature is me disappearing from the credits.
I then double checked, first to see if it was true and second to see if the game had changed so much that my contribution vanished.
Well, the game is still the same beside some art improvements.

I sent an email to my ex employer to know why. We used to have very good relations in the past so I was surprised to not receive any answer.

I would like to get some advice before taking any further actions.
Thank you.