If you don't know what else to do, start splitting the problem into parts. In this case, you might first investigate whether an attempt gets made to send the status report or not.

Look into /var/log/mail* logfiles: if you have enough older log versions saved that you still see the last successful sending attempts, find some useful search keywords, use the grep command to view only the log lines containing only those keywords. After that, you should be able to view the relevant log entries next to each other and easily spot the point where something changes and the reports are no longer successfully sent. What is that change?

If the attempts just stop, then it might be because there is a problem in the administrative task scheduler, or in the task that is supposed to send the reports.

Those tasks are executed by the "taskomatic" service. Is that service running?

Please see this email in the spacewalk mailing list. It might be relevant to the Satellite 6 too:

https://www.redhat.com/archives/spacewalk-list/2015-October/msg00025.html

You could also find the daily report task in the Satellite administration UI and see if it's been disabled for some reason.

If the attempt is made but the mail log indicates that the delivery of the report has started failing, talk to your mail server administrator to find out the reason why the mail server now rejects the reports, and the possible ways to fix it.