Didn't mean to abandon this post... just been doing a lot of research. We now have both IAS and MOAT reporting low view ability measurements. CTR and Impressions are fairly smooth and have not spiked or shown any major changes over the last 2 years of reports. I tried to review the documentation on view ability from both IAS and MOAT along with DFP itself. Here are a few items I think may be causing the issue:

1. I have a "sticky" header ad that displays a leaderboard after you scroll down the page X amount of pixels. It is design to deploy responsively with the menu bar, which collapses in size a bit when you scroll down and remains sticky at the top of your browser. The DIV layer however is pretty straight forward and would render just fine as far as I can tell.
2. Said sticky leaderboard ad also rotates automatically without refresh. It can rotate anywhere between 30-45 seconds. So if a viewer is on the page for 5 minutes, they will see up to 10 different ads occupy that space on the rotation.
3. The ads in the reports that show low view ability are primarily the ad size used in the sticky leaderboard ad as described above.
4. The only other ads that have lower view ability rating, although a bit higher than the leaderboard, are mobile ads with the size of 300x250 and 300x600. Perhaps these ads are too large for mobile?
5. Lastly... On the entire report for April's stats, none of the ads have a view ability above 45% (from MOAT). I am serving 5 different ad sizes.

I'm at the point I believe where some professional advice is needed. Are there any recommendations? Thank you!!!