No thumbnails on you tube videos.

I would be most grateful if someone could help me with the following.I watch a lot of motorbike racing,speedway in particular, on you tube.In order to watch exactly what I want and skip the rubbish, I move the video along using the little pictures that appear on the bottom as you move the dot along.I then know where to restart the video and can in effect watch a 20 race programme in about an hour, instead of having to sit through a video which is usually about 3 hours long.Unfortunately these pictures will not come on and I can find no way to make them come on.Can anybody help please?The laptop is an Acer aspire 5732z on windows 7.