I was going to say that rotation is the best solution for your problem, but then I read that you are rotating your crop. So the answer may be that you should plant your next set of bulbs as far away as possible from any of the places you've grown them before. It's also very important to get rid of badly affected plant material - throw it away rather than putting it on the compost heap. Finally, don't plant your garlic too close together. As with most fungal problems, humidity caused by the plants being close together will make the infection worse, and it'll spread more easily.
Have a look at Pippa's blogs -
even real experts can suffer from this problem!
I do hope you have better luck this year,