"cancelled appointment"

I have waited a while to get an appointment to see the diabetic consultant, after they said to me that I needed to see them in six months. a year has nearly passed. the appointment has been changed twice and today I received a call from the consultants secretary to say the appointment has been cancelled. The secretary couldn't give me a date for the next one. later on in the day I received a text reminding me to attend the appointment. I needed to contact the diabetic centre to check what was going on. I was told the appointment was cancelled and to ignore the text. this happens far too often at the diabetic centre. why are appointments changed or cancelled so frequently?