Without looking through the daily log files of the scan engines it is hard to tell what the hold up could be. If it appears packets are going out, I suspect memory allocation issues. If the scan engine does not have enough memory to launch a new batch it will hold the next batch waiting for resources until enough memory frees up. You can search for 'waiting for resources' in your log file to determine this. I suggest opening a Service Request to be sure. We maybe able to pinpoint the hold up.