[Lizard]

ivy3303: Select the milestone. Look in the task info inspector. If it says "on specified date", that's the problem. The critical path is the sequence of tasks where if any one of them slips, the chain of dependencies and other constraints causes the end date of the project to be delayed.
If the last task ends a week before the milestone, then it isn't on the critical path, because it can be delayed by a couple days and you'll still meet the milestone on time.
If this is the case, I recommend you reset the scheduling for the milestone. If there are dependencies pointing to it, then it will move as early as possible without violating those dependencies. This will show your critical path, but your milestone is now free-floating, so you'll want to add an end constraint to the milestone if you've got a deadline to meet.

I fear I may not be explaining this clearly enough. If you need more help, I recommend you email us (Send Feedback from the Help menu will attach a version of your file with task names and other info removed) so we can see exactly what's going on.