Indexing - limit on subheads?

[charles]

In indexing a document in Word 2007 I found that heads with subheads were
being repeated. That is, if I had a head "Boxes," and subheads "box1,"
"box2," "boxn," the index would look like this

Boxes
box1 p1
Boxes
box2 p2
boxes
boxn p3

When I cut a few of the entries so that I had a smaller number of entries
with that heading, the problem corrected itself and my index looked like
this:
Boxes
box1 p1
box2 p2
boxn p3

Is there really a limit in Word on the number of subheads under one heading
in an index? If so, is there a way around it?

TIA,
- charles thiesen