Why would a doctor not repeat an occipital nerve injection on a patient who had previously had one?

I suffer from frequent migraine and migraine clusters which have completley taken over my life. I also have chronic pain in my neck and shoulder which I feel contributes to some of my headaches. During a 6 week cluster I was given an injection of a steroidal and anasthetic nerve block injection. I was given the impression at the time that this could be repeated. The effect was huge offering great relief and 2 months of no headaches, migraines or neck pain. It has worn off and am now back in my familiar il state. My speacialists are refusing to re-inject me and has not explained why except to say that it wont work. Why is this? Can anyone enlighten me or offer me any alternative suggestions. I have tried physio & osteopaths for a long time with very limited success. I am on preventative medicine topamac as well. Iwould appreciate any ideas as I my own G.P. whilst trying to help, is at a loss.