Hyperlink to external bookmark

[pdb]

The scenario:
A.doc has
link1 {HYPERLINK "B.doc" I\ "bk_1"}
link2 {HYPERLINK "B.doc" I\ "bk_2"}
link3 {HYPERLINK "C.doc" I\ "bk_3"}
link4 {HYPERLINK "C.doc" I\ "bk_4"}

In A.doc if you:
click on link1 it opens B.doc and goes to bk_1.
click on link2 it goes back B.doc BUT STAYS ON bk_1
click on link3 it opens C.doc and goes to bk_3.
click on link4 it goes back to C.doc AND goes to bk_4.

C.doc has the desirable behavior, B.doc does not.
What is the difference?
How do you make B.doc act like C.doc?