Calendar showing time window as busy when it should be able to fit a service

I have two services defined -‘a’ 60 minute,’b’ 90 minute.

I have listed availability as from 19:45 – 21:15 i.e. we can do either ‘a’ or ‘b’ but only one a day.

When you look at the calendar it does not allow any ‘b’ sessions to be booked and shows that I am busy (even though there are no bookings) It allows ‘a’ sessions to be booked.

Only when I have adjusted the end of day to 22:00 it allows it to be booked BUT it only offers appointments at 8:30.

I am guessing its the appointment times causing the problem – Why does it not offer me 7:45 as an appointment time as thats when I am available.