Database - Not Live
Sonar is an extremely large database of product/keyword combinations. The Sonar database for Germany and USA are updated in roughly monthly cycles. For the other markets the last update was in April 2017, but will soon also be updated on a monthly basis.
If the respective product is relatively new on Amazon, it may not yet be part of the database.
Only Well Ranking Keywords
Sonar only includes keywords for which the ASIN is ranking on the first page of the searchresults (or at least was ranked at the time of the last database update).
The ASIN Reverse Search therefore offers a good opportunity to find out the keywords with which any given ASIN is generating most of its (organic) sales.