When you change tiers, your average is a clean slate - your average from the previous tier is wiped out so that your current average is based only on rounds in your new tier.
In terms of the average not changing, this is because your average is based on the best scores in X number of rounds. For Master, your average is based on your best 40 ranked rounds. If you have more than 40 scores, then rounds higher than your current average are "thrown out".
Hope that helps!