Not consistently showing full path in breadcrumb

I have a 3 level menu. Sometimes the breadcrumbs for a 3d level item are showing correctly:

a >> b >> c

However, some items do not show the middle level:

x >> z

rather than:

x >> y >> z

in each case, c and z are pages.

I can't see a pattern that would explain why sometimes I see the whole menu path and other times I don't.

Is there anything I should look at and/or try? I have already cleared the caches.

Thanks.