The most likely explanation is that there is some kind of filter working. When you look at the sheet, are there any fields on the filter shelf? Are there any data source filters being applied?
How do you know that data is missing? What granularity is your data stored at (e.g. is there one row for every Order ID, every item...?) View the underlying data and compare it to your original data source (the one that's telling you the Tableau number is wrong) to find out which rows are missing. Check to see whether the "missing rows" are really missing (i.e., not being pulled into Tableau) or whether they are just appearing somewhere you don't expect (sometimes end-of month transactions can be attributed to the following month instead of the current month, due to timestamp discrepancies).
If this helped resolve your issue, please mark your question as answered.
It's alway difficult to validate data by looking at a bar chart.
The best way to check your data is by creating a simple table in Tableau that shows you all the dates and the amount of sales.
In case you don't get the right result, try checking these things:
- Did you join the sheets from your excel file? If that's the case, double check all your joins
- Are you using a live connection to your excel file or are you working with an extract? In case you are working with an extract. Make sure that no new data was added to the excel after you created the extract
- Maybe you added some data source filters?
- It could be you added a filter to your view?
It's always easier for us to help you if you could post some images or a twbx.
I hope it gets sorted out!