@Nadeen1 -
I just checked in my playground, and the module locked with the date showing at the bottom (note this is in student view - there is no indication that it is locked on the instructor view-I'm blind the lock information is in instructor view as well - Not sure how I missed it the first time.).
If you were given the option to re-lock the module when you set the lock date, you need to select re-lock the module - just checked and that does not matter, but as a matter of principle I always relock a module when prompted.
If you are locking the module, but not seeing that it is locked in student view then there is something else going on.
Ron