Academy decals aren't that great. The most common problem is not getting them to stick or stay on. Use warm water with a few drops of Elmer's white glue and liquid dish soap. Make sure to apply decals over a gloss surface, but this gloss surface can be acrylic, enamel, or lacquer. Also make sure you soak the decals until you can move them from the paper backing. If they won't move, dip the decal back into the water for a little longer and try again.
You can contact Academy to get replacement decals. You can send them a letter, or you can go through their site, www.modelrectifier.com. You can also but aftermarket decals from any of the manufacturers available. www.greatmodels.com, www.squadron.com, www.aeromaster.com, and www.meteorprod.com are all great sites to buy from.
www.modelingmadness.com has a whole section of decal reivews which is helpful for seeing which ones you want to buy.