There are Open Registration Slots but the "Class is Full"
This usually means the class is set to: "Apply absolute Max regardless of class settings"
When creating or editing an activity or program, there is a "Class Size" setting, if you have it set to "Apply absolute Max regardless of class settings" it will limit your total number of registrations across all classes to the number you specify on the main program. You will need to change that setting to "Determined by the class settings". Once that has been changed, edit each of the classes and ensure the class size is set to the correct amount.