Calendar cancels an event after editing... what do I do?

Our office has a shared calendar (that I have shared) where all leave, meetings outside the office, office meetings and events are posted.

Every time an event is added or changed it comes up with my name regardless of which staff member issued the invitation.

If I try to edit an invitation (add attendees, edit time or details), the event gets marked as cancelled and attendees are notified that the event is cancelled.

Are there setting I need to change or is there something wrong with my iCal? Clients are getting annoyed with cancelled meetings (which haven't been cancelled!).