It seems to me like the bottle neck for blood donations in SA is the facilities: I have given at least half a dozen times in the past year or two and have always found it difficult getting an appointment (within a period of a couple of months). I get calls from the Australian Red Cross Blood Service suggesting there is an urgent need for donations, so I go to arrange a time and date within an (un)reasonable period: little luck, always fully-booked. So it seems to me like there is demand (obviously), there is supply, but there isn’t enough in way of facilities to facilitate that supply. Making an appointment straight after a donation seems to be the best way to go but I don’t tend to work like that - I tend to wait for a call and then consult my calendar within a space of a few months (so since I’m not working in the most efficient way I probably shouldn’t be complaining!)
My two cents (may well not be worth much).