Subform not appearing in report, but is part of the form

Subform not appearing in report, but is part of the form

I have a subform that is part of a main form. The subform appears on the main form and works. 

But when I open the report corresponding to the main form, there is no subform shown. 

What possible reasons could there be for the subform not to display on the report when it is visible
in the main form?

Sorry if this is a real basic question, but I am new at this. Thanks.