There's a feature inside of GHL, which if unknown, could cause serious issues with your campaign or workflow.

Depending on the package you have, there are various email sending limits, which if reached, will put a hard stop on your account.
It's critical to check that your emails are successfully being sent and received.

This can be found under 'Execution Logs' in the specific workflow.

Then, on the name of one of the registrants, hit 'View Details' to the far right.

If the window shows 'Success', then you're good to go.
If the window here shows 'failed', that's when you know there's an issue.

You can also find this issue illustrated here.

In any case, you should receive an email from GHL about this issue and should be addressed as soon as possible.