This is very strange. I've set up a bunch of 301 redirects from posts with user friendly names to dynamic urls with query strings. Of about 50 redirects, most work but half a dozen or so don't - they just show the empty post with featured image thumbnail. There are no duplicate post or page names (or media items) for the ones that don't work. See for example
darwinholidayapartments.com/home-on-hickory (doesn't work)
darwinholidayapartments.com/the-penthouse (doesn't work)
Any ideas why? All posts and redirects are set up identically.