The program is designed to be completed at a rate of one module per day. Once a module is completed, the next one will unlock at midnight.
Also, please make sure it's past midnight after you completed the previous module. For example, if you completed all the lessons in a module on the 13th of July at 02:30AM, the next module will not be available until 14th of July at 00:00AM.
NOTE: If you already completed some modules and they are now locked, please see this article My progress was reset to module 1!