I just did the flooding technique for the first time on sugar cookies. This morning I was so excited to get up and see how they dried overnight. I'm so disappointed! They didn't dry with the nice shiny sheen to the icing that I see in pictures. Can anyone tell me what I did wrong??

Put it under a lamp.

Usually royal icing dries to a matt finish. Those pictures are taken while the icing is wet. I do not know how glaze dries because I usually use royal icing.
Take a look at the one picture I have, the icing on the big hearts on the baking sheets are still wet and shiny and beautiful. On the other hand the finished cookie has a dull look to it.
I think that is how royal icing is. HTH

yes, glace definately dried with a shiny and smooth appearance while Royal icing dries matte.