flood of subscribers but no comments

A dozen new users register every day, almost 1000 now. From their email addresses I suspect that they are not actual readers (names that suggest sales opportunities. etc and many from Russia). This is a cancer blog so I get a certain reader profile. On the other hand no one ever seems to comment. One reader contacted me via email to say that she hadn’t been able to comment but she did not provide details.

I use Askimet and Si Captcha. This all started a few months ago when I changed themes, updated my version of WP from a very older one, and did a few other maintenance tasks