Monthly and weekly bonuses depend on the games that you play, the more you play slots with house edge the higher the rewards will get. The base amount is not actual anymore for VIP ranks, the wagered amount is the main thing that plays a role in the bonus calculation. Eddie confirmed this info a few months ago, so it is logical why bonuses go down each month.
As I remember your overall profit also affect the bonus amount, if you wager more and lose more means that you will get bigger bonus. Lets say you wager the same amount for week 1 and week 2, but your overall profit in these weeks are different (plus and negative). In this case you will get bigger bonus when your overall profit is negative. Correct me if I'm wrong.