Have you ever tried the IS critique forum?
As long as you don't come in with a rant (the inspectors are all blind; everywhere else these sell like hotcakes so what's wrong with IS...) you'll get a lot of specific and useful advice in spotting issues and how to fix problems.
If you do a lot of raster illustrations, that's an area with which IS has a love/hate relationship and other people have struggled with that too. There was a recent series of posts about some very insteresting fractal techniques that only got approved via Scout.
Each site has its policies, but once you've figured out what flies/doesn't, it shouldn't be too hard to get your work routinely accepted at IS.