What to check when validator is not called in symfony 3?

Question

I cannot find general checklist - what to check when it is not called. Can you write it?

For example code snippets where validator is not being called:

$fieldOptions['constraints'] = [
                    new NotBlank($constraintOptions)
                ];

$builder->add(
                        $builder
                            ->create($formField->getId(), EntityType::class, $fieldOptions)
                            ->addModelTransformer(
                                new EntityCollectionToArrayTransformer($this->registry, $fieldOptions['class'])
                            )
                    );