Products without Enough Sales / Only BSR in Main Categories
The salesrank is transmitted by Amazon's API.
Products without significant sales volume compared to other products in the same category usually do not yet have a salesrank in a main category. Sub categories are currently not taken into consideration.
Verify Amazon Product Detailpage for Salesrank in Main Cateogry
If the product page on Amazon does not show a salesrank in a main category (see below example: #22'774 in "Pet Supplies"), it is very likely the API too will not transmit a salesrank.
Very often it will only show salesranks in sub-categories. In the example below that would be #137 in the sub-category "Doors":