[Mailman-Users] how to force mailman send a message to all members at a time?

[surdet]

I have installed mailman version 2.1.12 with postfix. I have 6 members 
of a list. I found some time  mailman send a message to 4 members and 10 
minutes later it send a message to 2 members remaining. How can I force 
mailman send a message to all members at a time?

Thanks,
Mr.Suradet C.