Here is a thought.
What was the time frame between the second two boosts? If it was less than
three weeks, it is possible that the amount of circulating antigen-specific
Ig was high enough to form antigen-antibody complexes clearing most of your
immunogen out of the system. This would results in a reducing the actual
amount of immunogen left to stimulate the immune system to quite a bit less
than you might expect.
