I'm really struggling these days and I've just about given up trying. I can't book sessions far in advance because I'm not normally sure where I'll be working on any given day until a few days before, maybe a week at most. My attempts during the last couple of years at booking an appointment this far in advance have been unsuccessful due to all appointments being booked. Twice I've attended a session anyway and it was most unsatisfactory, I felt like a second class donor with my form firmly at the bottom of the pile with everybody who had appointments going on top. The reason this happens is because at each session people come to donors having their drink and biscuit inviting them to book an appointment in 3 or 4 months time. So, all the appointments are filled long in advance, possibly in many cases by people who have more flexibility about when they donate.
I've given this feedback to the service and I was phoned back but they're unable to offer a solution and frankly don't seem too bothered. Happily, it must mean they're not as short of donors as their adverts sometimes suggest. Hopefully I'll be able to start donating again when I retire.