To my knowledge, there is a few factors that could cause.
1 - Wagering - Obviously this is one of the biggest factors, but you said stayed consistent.
2 - Profit/Loss - This is an obvious important factor. If you are winning/doing well, I personally wouldn't expect the casino to hand me big reloads. On the FLIP SIDE, if i had sustained big losses, I would expect my VIP manager to look after me very well for that following week.
Those are the most important factors. Without knowing the full intricacies of how they calculate it, there could also be a rolling profit/loss over a ce