Imac problem with full screen web video

I have a relatively new 21.5" iMac. I recently I found that when viewing video content from the web, and trying to click on fullscreen mode, my dock gets hidden, but the web page with the video stalls and does not go to full screen. I first noticed this with Netflix movies, but then found that pretty much anything with video will not switch to fullscreen mode. Any ideas on how to fix?