Sounds like they're not OTS themes, I downloaded a lot of themes and the ones that are OTS work ok and the ones that aren't don't display anything at all, sounds like what you have

When you go into the theme switcher, hold the arrow/cursor over the theme you're trying to use...if no picture pops up in the preview window, it's not an OTS one (at least that's my guess with the problems I had)...the ones that have a preview shot work ok for me and the ones that don't have a preview don't work at all.