Yep it’s a total disaster the metrics have collapsed and revenue accordingly.
I’d up your 40% to 80% at times being down on metrics and revenue. 
This is also coming from a phpBB install.
Let that be a word of warning to anyone considering moving, needs to throughly understand the consequences and plan accordingly.
Impressions are wildly down, though we had probably over placed adverts to get that up, but it worked and the users never seem to complain despite it seemingly being overkill.
While googling this issue it’s apprenrlty similar also occurs for Wordpress (not sure if it wa plugin related). I briefly read something about Adsense not being able to find the right sized container and so it does not show an ad when this occurs. Is the same happening here?
I was making some mild CSS adjustments to .topic-body width and noticed when I did that seemed to push things wider and allow an advert suddenly pop into place. This advert was positioned at the bottom of a page, below the REPLY button array.
Is there something in the layout cascade and strucutrepreventing adverts appearing somethings. I have no idea. I could be assuming to much here and it could be coincidence. I’m simply noting what I observed.
Those more familiar with the way the pages are constructed and works together might have better insight.
It could also be a google thing where it doesn’t have the ad to display but it’s impossible to tell right now.
The plugin in my experience needs the following:
- better settings interface UI
- no. the ad units, helping cross reference.
- GFX/Icons would really help to indicate position of ad (settings)
- more positional controls or more positions to choose from.
- Sometimes it’s ok to have 3 ads in one row on part of a forum.
- Sometimes it’s ok to stack ads in 2 or more rows.
- nth ad on latest page would be a help too (with no next page it’s really needed).
- being also configurable separate to the topic nth.
Also to add, this is an early phpBB migration too and yes it’s understood traffic will drop (so no surprises here) but with redirects it hasn’t been as bad as expected on user traffic, getting very close to original pattern, maybe 10% lower daily user count (according to analytics) whereas adsense is very very very poor.
Maybe we made the mistake of making up a new ad unit for each entry/position in the adsense plugin (this was for easy of visible tracking for performance) but reverted to historical ad units after a few days. This helped a bit but it’s still 80% revenue drop problem.
Final note, adsense revenue trend was not great before, but this has made it worse so a portion of the issue could be changes on the google side. It’s hard for a Admin these days, so many things coming at ya! 