I wonder if the issue is that essentially, nearly everyone buys a vacuum at some point. So, you end up where the only distinct group of vacuum-buyers from whom

I wonder if the issue is that essentially, nearly everyone buys a vacuum at some point. So, you end up where the only distinct group of vacuum-buyers from whom you can drive recommendation behaviour is people who use Amazon only to buy vacuums, basically you're clustered into either the entire world or a tiny group of Amazon vacuum fanatics. Another issue might just be that the recommendation engine updates almost immediately, go to the homepage after taking any action (browsing, buying) and it's already updated. So they might've gone for "good enough, but fast" rather than "accurate, but slow"