Completed Tasks Not Moving to Completed Tasks
Some tasks are moving from the project list to the completed tasks list. Others are checked as completed but remain in the project list.
Please tell me - are all tasks you're referring to parent tasks? If a task is a sub-task, it will remain on the list until its parent task is completed in which case the complete structure will move to the history.
You can do this manually by right-clicking a task and choosing "Move to history" or, if you hold SHIFT while checking off a sub-task, it will be removed from the list automatically.