To reproduce..

• Follow the instructions here for adding a block to 'My Home Page' for all users, http://docs.moodle.org/24/en/My_Moodle#Adding_a_block_to_the_My_home_page_for_all_users
• In the final step there is only the option to set the block to the 'left' or 'right', not Centre
• it is possible to drag the block to the centre but this only applies to the admin user not to ordinary users who come to this screen

There should be some mechanism to default blocks to the centre column.