Concerns about mortgage pre approval from bank of america?

I received pre-approval from Bank of America for an FHA and VA Vendee loan. This was in early April and they pulled my credit before granting the pre-approval. I finally found a house and made an offer that was accepted. The proposed closing date was scheduled for July 29, but because of the length of time between then and when I received the initial pre-approval, I was advised to allow them to pull another credit report (this was last week). I did this and my score is now 656 (up from 628), my debt-to-income ratio has slightly lowered (due to paying off one credit card and paying down another) and everything else is the same. I had a bankruptcy which was discharged four years ago, and since that time I struggled for a few months (late 2008/early 2009) and had an erroneous state tax lien on my report. They asked from an explanation for these events and stated that I could provide a letter explaining the situations. These same items were on both credit reports pulled by BoA, but now I am being told that they are denying approval because of these items. Why would the grant a pre-approval for 170k, but deny my approval for 97k? I am a college educator and I have been on the job for almost three years. I was told that they would need certain items to process the loan, however they requested none of these items before denying me. I received pre-approval from Wells Fargo (also done in early April) which is still valid. I spoke with that loan officer today and he doesn't see the problem. I would prefer the Vendee program because of the lower fees/closing costs in addition to the zero money down payment, however I may have to go to the FHA from Wells Fargo. Does anyone have any suggestions on what to do or why this could have happened? Has this happened to others? Any information that anyone can give would be greatly appreciated. Thanks.