How many friends do you have, Manager? There is sort of an unspoken limit, so to call it. After reaching 100 friends, some may start disappearing from the Friends List/Fixtures. I see you are a Newbie, but if by any chance that's the case for you as well, try and clean up the list until you reach less than 100 again.

In any other case, the other manager could have unfriended you via FB, or their TE account was deleted.