Frank T wrote:
It's not that you did anything wrong, it's just the nature of photography. You exposed correctly for the people and that in turn overexposed the sky, burning it out. What you can try is to expose for the sky and then use flash to light the people so they're exposed correctly. Remember when you're setting exposure most cameras normally sync at 1/250 but many have settings that you can use for high speed sync; so check your manual.
Thank you FrankT, same thing has been happening to my photos with a 'big' sky background. I'm going out to try your tip now.