It's very common for sales to rise and fall over time, which will show up as periods of high volumes and low volumes in your downloads.
We can't comment on specific reasons why your download volumes might change over time or on other changes in customer behavior including redownloads, purchases, or refunds.
To access Sales and Trends, you need to have the Admin, Finance, or Sales role on iTunes Connect. If you're not sure you have one of these roles, check your role in Users and Roles. Here are the two main options in Sales and Trends:
If you see a message that your account is pending when you go to Sales and Trends, usually your updated user role is still being processed. If you're still seeing this message after 24 hours, let us know.
That item may not be available on the store. Make sure you've accepted the latest agreements in Agreements, Tax, and Banking. If you've accepted the latest agreement, check the Pricing and Availability section and the status for your item on iTunes Connect.
Yes, customer numbers provided in detailed reports are unique for each vendor ID and don't change over the lifetime of each customer account. To protect customer privacy, though, these numbers are anonymized.
Sales and Trends reports show refunds by product. Refund transactions will have a negative value in the Units field.