Email notification upon new og content intermittent or failing for some subscribers

Even when a user has their preferences set to "Always send email notifications" (in account settings > Organic groups settings > Email notifications), some users receive email only intermittently and some not at all. One of these users is one of my own test users, so I can confirm that the email is not going into a junk folder.

I've also confirmed that the og_email field is set to 1 in og_uid_global for the users experiencing this issue.

By contrast, when we use the manual og email tool (the third tab on an og's main page), emails go through to all subscribers just fine.

Is there a log file to see if an attempt was made to send the email? Has anyone else grappled with this issue? Thanks!