Differrence between regular fields and subform fields

Differrence between regular fields and subform fields

Hello, i use simple condition for checking some of fields in my form (located on edit > on validate). I find out, that my script works only for first row of subform.

if(Subform_name.Field_in_subform == null && input.Check == true)
{
alert "field in subform is not filled";
cancel submit;
}

How should it look like to make it work for all rows of subform ? Maybe its newbie question, if so, sorry for that.