everything that your subform is a child of has to page break, as well. So, your text field has to allow page breaks, as does your subform, any other subforms that the subform is contained in, and the page itself. Normally if you have a problem like this, there's something else that contains your text field that will have a yellow triangle on it with an exclamation mark, and it will say something like, "This object may not work properly. Although the object is allowed to break, deselecting the Allow Page Breaks Within Content option of the parent object resctricts this object from breaking between pages."
