My 4 years gal got chickenpox even though she is vaccinated. What can bethe reason? How to prevent this?

A. you have to give two doors age of chickenpox and you also have to confirm whether it is chickenpox or some other viral infection and after vaccination the immunity is around 80% to 90% so there is always a chance of 10% of getting the infection but the advantage the child would have is that the infection would be mild after vaccination