  We discuss the role of primes of good reduction in the existence of the
Brauer--Manin obstruction to weak approximation for varieties defined over
number fields. Following Bright and Newton, we give some necessaries conditions
on the ramification index that the prime ideal of the number field should
satisfy in order to be involved in the Brauer--Manin obstruction to weak
approximation. To support the results, many examples of transcendental
Brauer--Manin obstruction to weak approximation on K3 surfaces are given.
