There are a few reasons why sometimes you might see limited (or no) search results within Keyword Scout.
When you run a reverse ASIN search, Keyword Scout only returns the keywords that place that ASIN among the top 100 search results on Amazon. Just as the Best Seller Rank, every product has its keyword rank for a particular keyword, this helps with a listing position on the search results page. And the keywords that you see when running the reverse ASIN search are the keywords that rank is in the top 100.
This means that, even if an ASIN has 120 keywords, if the ASIN is not found in the top 100 results for any of them, then those keywords won't be displayed in the reverse ASIN lookup results.
You might not see any or get limited results when looking for specific keywords. In these cases, it is usually because the terms are relatively new or less popular than other keywords.
Results are tied to a specific marketplace. Searching for keywords in the Amazon US store will get you more results than searching in smaller-scale marketplaces such as Mexico or Germany.
Keyword Scout does a broad match on all keywords that share a certain number of ASINs with the original keyword entered, so it will give you more results than when running a reverse ASIN search. Once you get those results, you can use the filters to drill down and identify high-performing organic and sponsored keywords and keywords your competitors are targeting.