Homeopathic nosodes are sometimes wrongly referred to as a homeopathic vaccine. Nosodes are homeopathic preparations of the pathogen but this in itself does not stimulate the immune response in the same way as a vaccination.
Nosodes will cover the individual in the face of an outbreak of disease so that they will asymptomatically seroconvert in response to meeting the disease pathogen. The natural immune response will result in antibodies and memory cell production but it is as a result of being challenged by the pathogen itself and not as a result of the nosode. People who have given nosodes where a future titre test has shown antibodies present have mis-interpretted the meaning of this and have assumed that the nosode stimulated the immune response because they haven't witnessed the pathogen challenge that occurred during the nosode course because it was asymptomatic.
If an individual receiving a nosode does not encounter the pathogen at the same time, they will not seroconvert and will not have an immune response that conveys future cover. It is entirely possible therefore for an individual that has received homeopathic nosodes but not seroconverted to meet the pathogen in later life and become ill with that disease. It is not a failing of the nosode itself - just a failing of the individual to meet the pathogen whilst they were receiving the nosode. I hope that clears up a common misunderstanding.