Hi Yimha,
I have the same problem as yours. We can say that this problem is "normal" because we use the data grouping. When you group, you group the data you read, so if a date has no data, you won't have a group for it. It's logical.
My solution would be to create a new formula for grouping, with variables.
It would look like this:
For each date, if a record is in this date, datevariable + 1 (or amountvariable + invoice.amount).
The problem with this is that you will need a different variable for each day of the month and then do a formula to display each variable... this means a lot of formula...
if someone as another solution, please tell us!