You may need to update your client to the latest version, to do this the easiest thing to do is click the game window, hold the shift button and then press the f5 button. This should reload the game and client should be updated to the latest version. If you still have the problem check if you have lots of other programs downloading from the internet, this may also cause a problem. if the problem still persits, you can clear your cache or your cookies.  If you are using Firefox, you can do this by going to tools at the top of your toolbar, select clear recent history, and then  clear your cache.  if you are using Internet Explorer, you need to go to tools on the top of your browser, select internet options and then clear your cookies.  There is step by step instructions on how to clear your cache further down in our FAQ's