When you add a field to your form, the field is not considered as required (mandatory) by default. If the field must be filled each time the form is submitted, make sure to set that field as required.
To set a field as required, click on a field to select it, click on a Gear properties icon, then click on a Required control.
The same way the required status can be turned off.
Required ON = Field is mandatory and must be filled by the form user.
Required OFF = Field can be skipped by the form user.