"Booking an appointment 3 weeks in advance"

I’m pregnant and I’ve been told by my midwife I need an appointment at 31 weeks of pregnancy. I called them 2-3 weeks in advance which means they should be definitely able to give me an appointment, however the receptionist said because “they don’t have the rota” they can’t book an appointment in advance and to call again next week on Wednesday or a Thursday. Even if they don’t have a schedule, I have no preference as to the time or day. I don’t see why they can’t make a note of the fact that I need an appointment and when their rota comes in, just add me in and call me to let me know of the time. Instead I have to wait and call repeatedly on different days until they find an convenient time for them to make an appointment. This is not the first time this happened and I’ve been told that no appointments can be given in 2 weeks even more though I’m happy to book in advance, and no appointment available in the current week. So I have to go repeatedly and call repeatedly until they find it convenient to accept the fact that I requested an appointment. I think it’s absolutely unprofessional and unaccceptable.