When viewing cases in the FogBugz Iteration Planner or Kanban Board your users are not seeing all of the cases that they expect to be displayed, but when they search for the milestone that all of the cases belong to they can see all of the cases in the search results.
There is a 500 cases per milestone limit in the Iteration Planner and Kanban Boards. The system retrieves all of the cases for that milestone and displays the initial 500 cases. If you have cases that are not displayed, those cases did not fall within the initial 500 cases retrieved.
As a workaround, you can create a new milestone and move the closed cases or cases that you are not required to this new milestone. Once the milestone with the cases you want to be displayed has no more than 500 cases, the missing cases will be displayed in the Iteration Planner or Kanban Board.