Appointments + Notifications issue

Hi - I am having an issue with the appointments + plugin where the notifications do not appear to be going out to the customer or to the provider... I originally installed the free version, but have since upgraded to the paid (+) version. Notifications originally went out with the free version then stopped, then when I upgraded, they went out once when I tested it and have since stopped... Not sure where to go from here. Any help is appreciated. Thanks!