Missing or Incorrect Category/Browse Node: The most common cause. A BSR is assigned within a specific category or subcategory. If the product doesn't have a valid, deep-level category assigned in the backend (sometimes called the "Browse Node"), Amazon's system can't rank it against its peers.
New Product or No Sales History: A brand new listing that has not yet had any sales will not have a BSR. It must achieve its first sale for the rank to be calculated, and it often takes a few sales to register consistently.
Low Sales Volume/Velocity: If the product's sales are extremely low or inconsistent compared to others in its category, Amazon might not display a rank. The BSR only appears when the product is selling well enough to place in a category's rankings.
Product Flagged for Review or Misclassification:
Adult Product Flag: Products that are incorrectly or correctly flagged as "adult" by Amazon can sometimes lose their BSR and search visibility.
Misclassification: If the product is placed in a category that Amazon deems irrelevant, they can remove the BSR until it's corrected.
Variation Issues (Child ASINs): If the ASIN is a "child" in a variation family, and the variation relationship is broken, or the child ASINs don't have the correct matching browse nodes, the BSR may disappear.
Technical Glitch: Sometimes it's just a temporary bug in Amazon's catalog system, which may resolve itself or require a case with Seller Support to refresh the listing.
If you are the seller of this ASIN, you should check the following in your Seller Central account:
Product Category: Go to your inventory and check the Vital Info or More Details tab to ensure the product has a complete and accurate category/browse node assigned.
Sales: Confirm that the product has had recent sales.
Support Case: If the category is correct and sales have occurred, it's best to open a case with Amazon Seller Support (under "Inventory Uploads" or "Product Page Issue") and ask them to re-sync/refresh the BSR or confirm the current browse node.