I have been getting the same issues. It normally resolves itself by refreshing the page, but occassionally the only way to get it working is to clear browser cashe. This seems to be a new problem, was there an update recent update?
Are you using Internet Explorer? That is what I am running the game on. I have read that this is not the best browser to run the game in, although I don't have a choice because it is a work laptop.
