Easy Appointment Booking App

Customers were unable to make reservations, and we only discovered the issue three days before the event. Once a single booking was made, no further reservations could be placed by anyone else.
After contacting customer service, they appeared to resolve two of the three available dates by adding 100 slots per date. However, the third date was never fixed. Despite their assurance that it would be addressed, we received no follow-up whatsoever.
To make matters worse, the same blocking issue resurfaced on one of the supposedly fixed dates: as soon as one person booked, all subsequent reservations were again locked out.
We are deeply disappointed. The app is unreliable, and we have very likely lost customers as a direct result of these failures.