Is anyone having a problem with Youtube thumbnails not being displayed always?

I have noticed that YouTube thumbnails are not always showing on my website http://www.funkyhousevideos.co.uk/ . Also there not even being displayed on some google search results pages e.g http://www.google.co.uk/search?sourceid=navclient&hl=en-GB&ie=UTF-8&rlz=1T4GGLL_en-GBGB327GB327&q=problem+with+youtube+image . Does someone know why this is happening.