The program lessons are designed to be progressed one module per day. Once a module is completed, the next one will unlock on midnight.
Also, please make sure it's passed midnight after you completed the previous module. For example if you completed all the lessons of 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!