good stuff for one of your first zbrush sculpts! I'm not positive if its the problem, but i think you need to play with the gloss value of the garbage bag, if you're not familiar with gloss maps, just plug a constant into the specular power node in the material in UDK. I'd use a high value (tighter highilght), have a play…
Your spec looks extremely weak on the dumpster and fire escape thing. I saw the strip on the flats.. any chance for a larger image. It looks like they could use a levels clamp, and some tweaks of the spec power. Your diffuse maps look like they could use an unsharp mask as well. You could try lower resolution maps with a…