Adsense says it's because there are fewer ads in https.
Where did you read it? Because, if I don't make mistake, it's been a while that all Adsense ads have to be available through HTTPS. Isn't it ? And I thought that third party ad networks also have to comply with this requirement.
Do you see error messages about your ads ? (insecure connection)
A few months ago I noticed my ad revenues drop. My site was still largely HTTP only with HTTPS on pages that needed privacy/security (login, messages). I took the opportunity to update my Google DFP tags then and my revenue went up (AdSense as fallback) in most inventory. The new tags were HTTPS.
I moved my whole site to HTTPS now and revenue if nothing got better. The server sends header policies to make sure only HTTPS elements are loaded (no mixed content warnings) and I made a point of making sure every single piece of script was served over HTTPS.
Perhaps your site still has some old scripts that need updating? Perhaps the response headers are too strict and only allow some domains to load? You might have to look at this kinds of things.
"Not many HTTPS ads" is not an excuse. AdSense has been serving HTTPS ads for years and advertisers know well how to manage this.
Moving to HTTPS is about security so you shouldn't make it less strict, but check that your site sending the correct scripts, images, etc - all using HTTPS, not HTTP.
The best thing to do is to start with Developer Tools (in Chrome SHIFT CTRL I) and see if anything fails to load on your page. Work from there. If everything loads ok then it's something else.
Also are you correctly redirecting (301) from HTTP to HTTPS or just dropped HTTP responses? Have you added your HTTPS domain to Google Webmaster? Remember https://www.example.com is different than http://www.example.com.
There are so many things to check...
[edited by: not2easy at 2:51 am (utc) on Jun 5, 2017] [edit reason] readability [/edit]
Thank you. In Chrome I did SHIFT CTRL I and I get these errors, could any of them help to explain a problem? Are these problems for Google?
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