Queue is not processed, no errors

Hello everyone,
I have recently had a problem with processing the queue. When I send a message to several recipients, the queue is not processed. The test message to one recipient works without any problems.

I have define(‘VERBOSE’, true); and error_reporting(1) activated. Unfortunately, no errors are displayed because the queue does not come to an end. It runs and runs and runs, I have to cancel manually.

How can I find and fix the problem?

Thanks in advance!
Greetings Andi

processing queued campaigns