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hit the button that has a circular arrow on it in Safari on your tool bar. It starts life as a cross while the page is loading and then turns into the circular arrow.

If you're still having trouble then Empty Cache (under Safari menu) might also help. I have to do that when using some travel sites as a simple refresh doesn't do the trick.
 
You can also go to View > Reload Page or press command R. And while we're talking about command R, command [ goes back a page and command ] goes forward a page.
 
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