That would be my guess too. The tendency is to set the flash to vapourize; thus, flooding the subject with
light and eliminating the natural balance of
light.
What you want to practise doing is setting the flash to the minimum needed to fill the shadowy areas (having as little effect on the lit areas as possible). You need to learn to deal with the slowest
shutter speed you can. If its a party for eg, maybe 1/60 (it will vary). If the flash is set low enough to just fill the shadows and to still allow the ambient
light to be seen, that is double plus good.
Hope this helps.
Scotty