"Send email" action not working to send email to poster of comment
This question can be very hard to answer -- email is tricky as PHP just hands off your email to the underlying/remote mail system. I suspect nobody can tell you exactly what's wrong without in-depth examination of your setup.

But, the Mail Developer module will allow you to "try" to send and log raw emails without actually sending them, you can likely look for differences between mails that do send -- and the emails you think are not sending correctly.


  Maillog provides an easy possibility to log all Mails for debugging
  purposes. It's possible to prevent the mails to being sent, so there
  is no need for an extra mail server to test the mail functionality of
  other modules or the drupal core. Additionally you can immediately
  display the mail through the devel dpm() facility.
  
  If you like to upgrade from a previous dev release, please disable,
  uninstall and reinstall again. There's no upgrade procedure yet.


Also see the similar modules on the project page -- for additional tools to debug and resolve your problem.