Since we use Amazon as our source of data, and the data of some books on Amazon do have some irregularities from time to time (e.g. no price displayed or no best sellers rank displayed) in these special cases we cannot show you the data points that Amazon does not display.
There are also may be cases where Amazon has not yet assigned a BSR for a book. Usually, this is because it has been recently published and has not yet made any sales.
If you are encountering an issue with BSRs not appearing, you may find this article helpful.