Why can't more than one person sign-up for an appointment?

Here's the site in question...

http://www.lowercolumbiacap.org/energy-assistance-sign-up/energy-conservation-class/

I have it set on the backend so that 30 people is the capacity for this appointment. However, when one person books the appointment that time slot becomes unavailable so no one else can register.

Any ideas why this would be happening? 30 people should be able to book that a time slot before it is unavailable.