@pancakehollow If you look at the Event Log page then you should be able to see how many emails phplist is sending in each run of the cron job. I guess it is much less than 100 and possibly sometimes 0.
Because of the way that phplist works out how many emails to send in the next batch, the batch period needs to be reduced to a small value. See my explanation at Struggling to get cron job timing corect - #2 by duncanc