When availability appears to stop working, it's likely one of two things. The most popular cause is that you've turned off availability. Just click the Availability Button so that it says, "Availability On."
If availability is on, but you still see no availability meatballs, it's likely that your Start Date and End Date are in the past.
If you see the meatballs, but they're all yellow with padlocks, there's a downstream document (i.e., a Pull Sheet or Manifest) that is locking availability.
See Availability Explained for more information about how availability works.