Apple GameCenter start question...

I have included GameCenter in our new game but noticed it sometimes can be accessed from inside the app and sometimes not. When it can’t be accessed the only way to “fix” it is to click the home button and then bring back the app on screen again. Then it can be accessed.

Is there a way to do this kind of “making it work” form inside the app by code somehow?

The same problem seems to appear when working with other game networks, like the Google one.

Any idea?