AngularJS validation does not work very well

I have a form that is shown within a modal, used by the Bootstrap framework, and because of this, angle validation does not work right. Only a few fields are validated and still random.

For example, I have three fields in this form: FirstName, LastName & Email. Sometimes the email field is validated and the name too, if I close the modal, that fields are no longer validated. And so it goes.

Would anyone have any tips that could solve this?

The validations are: Email must be of the email type and mandatory field Fields first and last name must be mandatory only.

The code for my form:

          <form class="ui form" name="add_usuario">
            <div class=" required field">
                <label>Email</label>
                <input type="email" ng-model="email" name="email" required>
            </div>
            <div class="required field">
                <label>Nome</label>
                <input type="text" ng-model="nome" name="nome" required>
            </div>
            <div class="required field">
                <label>Sobrenome</label>
                <input type="text" ng-model="sobrenome" name="sobrenome" required>
            </div>
            <div>
               <button class="ui blue ok button" ng-class="{'loading': enviando}" ng-disabled="add_usuario.$invalid || enviando">Salvar</button>
            </div>
        </form>

It's worth noting that this validation just does not work when it's within the modal, to put it directly on the page, this validation works without any problem.