Generally the best converting affiliate sites are the ones that rank for brand names (think shopping and coupon sites). Someone searches for a brand name (say Amazon), if your site ranks on a search for Amazon and offers them a coupon then the click to sale conversion rate from your site to the merchant site can be 25+%.
The site captures someone with strong intent to purchase, offers a coupon or just presents a good price from a reputable merchant to offer a little extra nudge and the sales start rolling in and you start making money.
If you can rank for "Brand X Shoe Model" and someone does that search, chances are they are looking to buy that specific model of shoes but don't yet have a specific store in mind to buy them from. If you can serve up a variety of places to buy them with a few options so someone can pick what they think it a good deal the conversion rate will probably be lower than someone searching for Amazon that you pull in but will probably still be pretty good.
If the search is just "Brand X shoes', then the consumer probably isn't quite as sure about what they want to buy as the person who searched for a specific model. If you can give them enough ads or products to choose from to make a decision, some sales will be generated but chances are the conversion rate for the "brand X shoes" will be lower than "Brand X shoe model"
As the search traffic that leads to a site gets farther away from a specific store, brand or model of something that someone wants to buy and becomes more generic the conversion rate tends to drop dramatically.
With affiliate ads, the site has to pull in people with intent to buy and the ads need on the page need to offer up a path to buy whatever the person wants to buy. If not it is just free advertising for whoever the merchant is that the site is running affiliate ads for.
If the traffic to a site is not capturing people with strong intent to do something now, it is probably better off running ads that pay per impression or per click.