increasing art for reproduction can only go so far ... generally a 3 pixels to 5 pixels increase still has good image quality, but when you pass that it starts to look chunky to the point of being unprintable. Amongst some of the complications is that images become less crisp, image becomes less defined (and there is a difference ... most notably defined shapes become blurry and solid lines will get jagged being the main differences in the two), and noise will develop in soft areas.
Not to say you can't do anything about it. If you were going for less accurate representation of the materials, you could always add noise to the image before printing, increase the levels to help further seperate the middle tones to highs and lows, make things haftoned, use your unsharp mask filter, desaturate > gaussion blur > levels and make sure it's posterized, etc. All these make for art that isn't true to the original, but can help avoid getting better art (which is not always available or beyond the client knowing how to get or actually caring about getting for you)
if it's something you really don't mind going back into and futsing with ... you could always drag your wacom out and select your smudge tool to selectively put definition back into the art and clean up some of the chunkiness that developes ... takes longer, but helps a lot.
Oh and always make sure to work in layers.