How use a property of child page for check box caption?

I can use a property in same page as the section context as check box caption in the section.

E.g. in a section run under pyWorkPage, I can use a property A (type: text) in pyWorkPage as check box caption.

.A

But If there's a child page said PageA defined under pyWorkPage, then i use a property B defined in PageA as check box caption, error reported that page name not found.

.PageA.B

Even I define .PageA in "Pages & Classes" it does not work.

thanks.