Some doctors wouldn't know a bad reaction to a medicine or vaccine if it hit them in the face. Some doctors are observant and cautious. Some however run assembly line practices and if {the reaction} it's not seen they never will believe it. If it's not in their little manual of probabilities they proceed to overlook the part of possibilities and just don't take the needed time address the issue.
Of course these type doctors rarely do see or believe it as the patient or parent usually goes to another doctor who will listen and do some detective work.