Mailgun gives cron lock

Every time I switch over to mailgun plugin I get Cron Lock. All parameter have to be so low that it’s hopeless to use this. I have to set the number of email to 20 and 150ms between each and then is’s working for 2 or 3 cycles. I use cron job on siteground to run the job and this is configured to run every 30 min. So my newsletter use 2-3 weeks to get out. I never gets cron lock when using simple smtp.