8 листоп. 2019, 15:4108.11.19
You might have already done this, if not,you might get some idea if you read old posts, but again old posts might not be so relevant now.
We dont really know as we cant see what you are doing, and its never officially stated as to how, what, where, and when.
It is likely that is it is connected to how often you buy ,and what you buy, and when you last bought, and what your palace level is.