The power of Dynamic product ads is that the ads are custom to the person looking at them, which means less work spent updating ads. But with that is a tradeoff in control over what shows in the ad. If the items have been viewed, they will be shown to that customer. One thing to keep in mind is that this campaign type only targets people who viewed or added items to cart but did not purchase.
The ad creative is pulled directly from your store's product catalogue and right now we do not have a way to restrict what products are shown. Facebook does show the best performing ads at the beginning of the carousel, so if your lesser known products are not getting as much engagement, they will show towards the end of the carousel.