Unless you went for H/W/S and then one of the MIT, Kellogg, GSB route then I dont think its academics. Plenty of folks get into school with worse GPAs and similar/worse GPAs from schools without the rep of U Mich. Your GMAT is slightly below average for top schools, it probably would be wise to try to raise that into the 720+ range. I think that your lack of success has more to do with other parts of your application. It also could be a problem with the schools you choose.
If you dont have a very strong quant background...meaning you got below B's in your quant classes in college then your chances at quant heavy schools will be a little worse. MIT, Wharton, GSB all are going to attract lots of very strong quant folks. Personally for these schools I think no quant is better than below average (C or less in classes).
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Kellogg Class of 2010...still active and willing to help. However, I do not do profile reviews, don't offer predictions on chances and am far to busy to review essays, so save the energy of writing me a PM seeking help for these. If I don't respond to a PM that is not one of the previously mentioned trash can destined messages, please don't take it personally I get so many messages I have a hard to responding to most. The more interesting, compelling, or humorous you message the more likely I am to respond.