Once all dates in a plan have passed, the plan is no longer showed on the main dashboard. You can load past plans from a Service Type.
Click the Upcoming dropdown to show options of the past plans that you can view. Choose one of the preset options or Custom.
If you choose Custom, you can set a date range of past plans to view.
If you have a plan displaying that should be archived, it means there is a date involved with that plan that has not yet passed (usually a rehearsal or other time). Go into the plan, click each time in the times section in the upper left, and make sure all the dates are set correctly. Many times a date has been set a year in the future by mistake.