Not all viruses are created equal; some are much more prone to mutation than others. And indeed, vaccines for highly mutation-prone ones do "fail" in some sense -- that's why we need flu shots every year but one or two doses of the chickenpox vaccine are preventative for a lifetime: the flu is an RNA virus and also has a mechanism for swapping genes between different strains infecting an individual organism (including animals), so it changes a lot, and vaccines only last until the in-the-wild flu changes out from under them, whereas chickenpox/varicella is a DNA virus that's much more genetically stable.