On donation day, we send the payments we've received from the retailers to you. If the amount is less than expected, it may be because some donations are still pending or confirmed but haven't been paid by the retailer yet (not ready for payment). We have more information about this here: Donation Day Payments Explained.
You can keep track of the total amount ready to pay on your transaction summary page. You can find this by going to the causes page, selecting your cause, and then selecting View all donations under the "Donations Raised" section.
Other ways to phrase this question
Why the amount of donation we received less than we thought
Why is the donation received less than it should be
I was expecting more donations to be sent to us