That the date is not changing to the date you want is telling you that the
way you have planned the work will not meet your objectives. In order for a
milestone to fall on or before a certain date it must be driven there by the
events that lead up to it. If you see it falling early, fine ... that means
you've got enough time left to meet your objectives doping the predecessor
work the way you have planned it. But if it's late, that means you must do
something in the path leading up to it to make that path shorter - rearrange
the work or find someone else to shoulder part of the workload, that sort of
thing. Remember, other then the project kick-off date you should normally
never, ever input the dates where you think things will take place or where
they need to take place - you don't tell Project the dates you want, it
tells you the dates you can have. If the result doesn't fall into your
business needs, you need to "tweak" the project's structure, workflow, and
resource assignments until it does.