Appointments+ implementation - no timeslots are green

I've installed Appointments + and all seems fine except that my front-end calendar shows all timings as red ("not possible"). I have noticed one of the FAQs lists a problem roughly similar to this one (but it appears to be more for cases where the timeslot is displayed as "free" but is unresponsive to clicking). That doesn't quite reflect my problem but I went ahead and tried the suggested remedy for that – which was trying different themes and disabling plugins. None of this fixed it. Is there a setting I'm overlooking or some other suggested solution?