You will need to carefully read the instructions and the request for evidence letter. If you are missing some parts of the tax return documentation, then you may face a possible denial. Perhaps, you should seek an appointment with a competent and experienced immigration attorney, who can take the time to review the USCIS request before something more serious happens.
Perhaps, something very simple, like a W2 is missing, but this is unclear.
This is general information, not legal advice, and does not create an attorney client relationship.
Did you submit you affidavit of support at all? From what you describe, it appears to me that you only mailed your tax returns, but not the affidavit itself. I would suggest that you take a copy of your application to an immigration attorney for review. If you do not respond to the request properly, your application will be denied and your fees will not be refunded.
Nothing in this post shall be construed as a legal advice. If you need advice regarding your particular situation, please contact my office directly. This information is transferred without an intent to creat an attorney-client relationship.