Infection Control and former Public Health Nurse here.
Once you have had chicken pox, the virus retreats into a part of the nervous system called the basal root ganglia. If you are lucky, it stays there, dormant (sort of asleep) for the rest of your life. If you are unlucky, it becomes active again at some point and travels along the nerve that is served by that basal root and causes numbness, itching, pain and blisters along the area served by that nerve. That's what is called shingles. Anyone who has had chicken pox in the past can get shingles.
No vaccine is 100% effective. The
main purpose of the chickenpox vaccine is to prevent people from getting chicken pox at all. One of the things that they found on studying people who had received vaccine is that those who had received the vaccine had less severe cases of chicken pox if they did actually get it.
There are a lot of reasons why someone might not be 100% protected after receiving a vaccine (besides that no vaccine protects everyone). One reason might be factors about the person themselves (like is their immune system working at total effectiveness). Another factor might be the storage of the vaccine. The varicella (chicken pox) vaccine is very picky about
how it is stored -especially temperature and not going thru any freeze/thaw cycles. If a lot of people from the same area got vaccine and still got the disease, one of the things to look at would be whether they got their vaccine at the same place and whether it was/is being stored correctly.
Here is some factual information:
CDC (Centers for Disease Control) website - General FAQs regarding varicella (the virus that causes chicken pox and shingles)
CDC website - General FAQs regarding shingles.
CDC website - Chicken pox Vaccine related FAQs.