If a object should allow page breaks then all it's direkt parents also hat to allow this.
As an example: If the nested subform "subform1_2_1" should allow breaks then its parents have to be flowed and allow page breaks too.
form1
pageset
masterpage1
page
subform1
subform1_1
subform1_1_1
subform1_2
subform1_2_1 (allow page breaks)
subform2
subform2_1
subform2_2
Hope this helps!