Milestones not completing

I am becoming quite frustrated now :

I have set up various milestones in a project, both internal and external, and am progressing the tasks within the task lists associated with the milestones.

I have one specific instance where I have a milestone which had a task list with a single task within that task list; that task was completed on time, the task list is effectively complete, but the milestone shows as overdue and I can find no way of forcing completion of the milestone

I have since done some testing and find that milestones never seem to complete - which as far as I am concerned seems to invalidate a key piece of functionality

Please advise