I have been wondering about refreshing native ads as well. The problem is that once an ad is loaded into a tableview it remains there for the life of the session - even when the tableview is refreshed, or the app is sent to background and then brought back to active. This makes monetizing from native ads almost impossible.
Two possible solutions: new native ads are loaded by a timer the same way that a banner ad is refreshed, or they are refreshed each time that the tableview refreshes.
As it is now, in my development version about three ads are loaded when the tableview loads, and despite the tableview refreshing every 500 m, or 5 minutes whichever come first, the ads are the same. Thus over many hours or even days of use the user only sees three ads.