So I've been running Google Adsense on my website for a few months now. I've always used a simple responsive ad block for both desktop and mobile. However, I noticed that the ad doesn't always scale correctly, so I chose to add an 320x100 ad unit and use this as a replacement for the reponsive ad block on mobile devices. So now on mobile devices, I show the 320x100 ad unit and on lager screens I show the responsive ad block.
Most of the times, Google Adsense can't provide me with a 320x100 ad though. Why does this happen? Why can't Google Adsense find an ad to show in the 320x100 block? I only get an ad like 1 in 10 times.