You probably don't need to use PMFSendMessage() in a trigger. There is an option in the Properties of tasks to send an email to the next assigned user. See:
https://wiki.processmaker.com/3.2/Tasks#NOTIFICATIONS
If you do need to use PMFSendMessage(), then you should run a case with
Debug Mode enabled to see if there is an error message in the __ERROR__ variable. If not, then go to
Information > Notifications to see if the email was created. If you see the email in the list, but you didn't receive it, then you probably need to set up a cron job in Linux or Scheduled Task in Windows to
periodically execute your cron.php script to send out emails.
Also check the return value of PMFSendMessage():
@@ret = PMFSendMessage(...);
If set to 0, then the email wasn't set, then you have a problem in your parameters. It could be many things, such as:
1. bad email addresses,
2. wrong spelling of the email template filename,
3. the email in the $from parameter isn't the same as the email account you used under Admin > Settings > Email Server,
If you still can't figure it out, then export your process and post the .pmx file here, and I will look at it.