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The sites may be PR4 or above, but maybe the link pages aren't. (therefore they won't show) - remember PR is page rank, not domain rank. If the links page is 2 levels down, their is much less PR on the page.
Also the people who traded the links with you may have robots.txt that excludes their links page. Some shady characters will do that
They may have cloaked their pages to present google with different info than what the average user sees.