I am a developer that supports a group of not-for-profits at my own expense. On two of the sites cfcpa.org and cpr-fl.org, the newsletter plugin will not let the cron run. I can send test messages but I cannot get the plugin to automatically send out as it did before. These sites have been successfully using Newsletter for several years. About 6 months ago, I upgraded the license to premium. within the last month, the sites stopped working. On the third site, thesludgereport.org, I have no issues at this time. All sites run the same version of WordPress 5.3.2 and the same version of PHP, 7.3. I did try one site on 7.2 of PHP but it made no difference. My testing has been on cpr-fl.org. On this site, I was able to use advanced cron manager plugin to force the process of sending email forward. I also tried to reinstalled WordPress. It did not correct the problem. Basically, the error with WP Scheduler remains in spite of my remediation steps.
I am looking for advice as to how to get the cron working again. I am assuming that since the plugin worked for such a long time, an update of some type may have introduced a bug. Can someone assist me with this? I do use IP blocking plugins for security but even when these are off, the cron error message remains. I use these same plugins on the third site also that does not display and error.
I will pursue the issue more. Just adding to this posting: I can use the advanced cron manager and force the newsletter plugin to run. It runs for 5 newsletters (set at 200) and then stops. After many successive starts by the cron manager, the newsletter is successfully sent. Cron seems to work but the plugin does not seem to be initiating or sustaining the activity. If I can find more or resolve the issue, I will add to this post.
Added a Curl command through my cPanel for cron. Used your suggested command from documentation. What I notice is that the error message is still active on my dashboard. Since my plugin and site worked well both with the free version and the premium upgrade until upgraded late last year, I think that there is something in the plugin that has changed. The fact that I can manually run it tells me cron works when using the advanced cron manager but when I let the plugin try to start it, it does not.